# Quelle:Ry/Accelrys Inc. - Forcefield-Based Simulations 1998

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 Autor Accelrys Inc. Titel Forcefield-Based Simulations Ort San Diego Datum September 1998 Anmerkung http://www.esi.umontreal.ca/accelrys/life/insight2000.1/getdown.html URL http://www.esi.umontreal.ca/accelrys/pdf/ffbs980.pdf Fragmente 11

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II.1. Molecular dynamics simulations

Typical use of molecular dynamics (MD) include:

- Searching the conformational space of alternative amino acid side chains in site-specific mutation studies.

- Identifying likely conformational states for highly flexible polymers or for flexible regions of macromolecules such as protein loops.

- Producing sets of 3D structures consistent with distance and torsion constraints deduced from NMR experiments (simulated annealing).

- Calculating free energies of binding, including solvation and entropy effects.

- Probing the locations, conformations, and motions of molecules on catalyst surfaces.

- Running diffusion calculations.

In addition, simulation engines can be routinely used for:

- Calculating normal modes of vibration and vibrational frequencies.

- Analyzing intra molecular and inter molecular interactions in terms of residue-residue or molecule-molecule interactions, energy per residue, or interactions within a radius.

- Calculating diffusion coefficients of small molecules in a polymer matrix.

- Calculating thermal expansion coefficients of amorphous polymers.

- Calculating the radial distribution of liquids and amorphous polymers.

- Performing rigid-body comparisons between minimized conformations of the same or similar structures or between simulated and experimentally observed structures.

At its simplest, molecular dynamics solves Newton’s equation of motion:

$\mathrm{F}_i(t)=m_i \, \mathrm{a}_i (t)=m_i \, \frac{\partial ^{2}\mathrm{r}_i}{\partial t_i^{2}}$ (2.1)

where Fi is the force, mi is the mass and ai is the acceleration of atom i.

Molecular dynamics

Typical uses of molecular dynamics include:

• Searching the conformational space of alternative amino acid sidechains in site-specific mutation studies.
• Identifying likely conformational states for highly flexible polymers or for flexible regions of macromolecules such as protein loops.
• Producing sets of 3D structures consistent with distance and torsion constraints deduced from NMR experiments (simulated annealing).
• Calculating free energies of binding, including solvation and entropy effects.
• Probing the locations, conformations, and motions of molecules on catalyst surfaces.
• Running diffusion calculations.

Other forcefield-based calculations

In addition, simulation engines can be routinely used for:

• Calculating normal modes of vibration and vibrational frequencies.
• Analyzing intramolecular and intermolecular interactions in terms of residue-residue or molecule-molecule interactions, energy per residue, or interactions within a radius.
• Calculating diffusion coefficients of small molecules in a polymer matrix.
• Calculating thermal expansion coefficients of amorphous polymers.
• Calculating the radial distribution of liquids and amorphous polymers.

[page 4]

• Performing rigid-body rms comparisons between minimized conformations of the same or similar structures or between simulated and experimentally observed structures.

[page 192]

At its simplest, molecular dynamics solves Newton's familiar equation of motion:

$\mathbf{F}_i(t)=m_i \mathbf{a}_i (t)$ Eq. 83

where Fi is the force, mi is the mass, and ai is the acceleration of atom i.

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The force on atom i can be computed directly from the derivative of the potential energy V with respect to the coordinates ri:

$\mathrm{F}_i=-\frac{\partial V}{\partial r_i}$ (2.2)

The classical equations of motion are deterministic, which means that once the initial coordinates and velocities and other dynamic information at time t are known, the positions and velocities at time t + Δt are determined (calculated). The coordinates and velocities for a complete dynamics run are called the trajectory. The time step Δt depends on the integration method as well as the system itself. Time step should be small enough in order to avoid integration errors. Because of fast vibrational motions of the atoms, a time step of 1 fs is usually used. Although the initial coordinates are known, the initial velocities are randomly generated at the beginning of a dynamics run, according to the desired temperature.

Analytical integration of equations such as (2.1) is not possible for large systems, so numerical integration should be performed. Molecular dynamics is usually applied to large systems. Energy evaluation is time consuming and the memory requirements are large. To generate the correct statistical ensembles, energy conservation is also important. Thus, the basic criteria for a good integrator for molecular simulations are as follows:

• It should be fast, ideally requiring only one energy evaluation per time step.
• It should require little computer memory.
• It should permit the use of a relatively long time step.
• It must show good conservation of energy.

Variants of the Verlet [24] algorithm of integrating the equations of motion (Equation (2.1)) are perhaps the most widely used method in molecular dynamics. The advantages of Verlet integrators is that these methods require only one energy evaluation per step, require only modest memory, and also allow a relatively large time step to be used.

24. Verlet, L. Phys. Rev. 1967 , 159.

The force on atom i can be computed directly from the derivative of the potential energy V with respect to the coordinates ri:

$- \frac{\partial V}{\partial \mathbf{r}_i} = m_i \frac{\partial^2 \mathbf{r}_i}{\partial t^2_i}$ Eq. 84

Notice that classical equations of motion are deterministic. That is, once the initial coordinates and velocities are known, the coordi-

[page 193]

nates and velocities at a later time can be determined. The coordinates and velocities for a complete dynamics run are called the trajectory.

[...]

Given the initial coordinates and velocities and other dynamic information at time t, the positions and velocities at time t + Δt are calculated. The timestep Δt depends on the integration method as well as the system itself.

Although the initial coordinates are determined in the input file or from a previous operation such as minimization, the initial velocities are randomly generated at the beginning of a dynamics run, according to the desired temperature. [...]

Criteria of Good Integrators in Molecular Dynamics

Molecular dynamics is usually applied to a large system. Energy evaluation is time consuming and the memory requirement is large. To generate the correct statistical ensembles, energy conservation is also important.

Thus, the basic criteria for a good integrator for molecular simulations are as follows:

• It should be fast, ideally requiring only one energy evaluation per timestep.
• It should require little computer memory.
• It should permit the use of a relatively long timestep.
• It must show good conservation of energy.

[page 195]

Verlet Leapfrog Integrator

Variants of the Verlet (1967) algorithm of integrating the equations of motion (Eq. 84) are perhaps the most widely used method in molecular dynamics. The advantages of Verlet integrators is that these methods require only one energy evaluation per step, require only modest memory, and also allow a relatively large timestep to be used.

Verlet, L. Computer experiments on classical fluids. I. Thermodynamical properties of Lennard-Jones molecules", Phys. Rev., 159, 98-103 (1967).

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Verlet Leap-Frog algorithm requires r(t), v(tt/2), and a(t), which are (respectively) the position, velocity, and acceleration at times t, tt/2, and t, and compute:

$v \left ( t + \frac{\Delta t}{2} \right ) = v \left ( t - \frac{\Delta t}{2} \right ) + \Delta t \, \mathrm{a} (t)$ (2.3)

$\mathrm{r} (t + \Delta t) = \mathrm{r}(t) + \Delta t v \left ( t + \frac{\Delta t}{2} \right )$ (2.4)

$\mathrm{a} (t + \Delta t) = \frac{\mathrm{f}(t+\Delta t)}{m}$ (2.5)

where f(t + Δt) is evaluated from -dV/dr at r(t + Δt).

The leapfrog algorithm

The Verlet leapfrog algorithm is as follows:

Given r(t), v(tt/2), and a(t), which are (respectively) the position, velocity, and acceleration at times t, tt/2, and t, compute:

where f(t + Δt) is evaluated from -dV/dr at r(t + Δt).

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AMBER Force Field

The AMBER energy expression contains a minimal number of terms. No cross terms are included. The functional forms of the energy terms used by AMBER are given in Equation (2.6).

Standard AMBER forcefield

The AMBER energy expression contains a minimal number of terms. No cross terms are included. The functional forms of the energy terms used by AMBER are given in Eq. 19.

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The first three terms in Equation (2.6) handle the internal coordinates of bonds, angles, and dihedrals. Term 3 is also used to maintain the correct chirality and tetrahedral nature of sp3 centers in the united-atom representation. In the united-atom representation, nonpolar hydrogen atoms are not represented explicitly, but are coalesced into the description of the heavy atoms to which they are bonded. Terms 4 and 5 account for the van der Waals and electrostatic interactions. The final term, 6, is an optional hydrogen- bond term that augments the electrostatic description of the hydrogen bond. This term adds only about 0.5 kcal.mol-1 to the hydrogen-bond energy in AMBER, so the bulk of the hydrogen-bond energy still arises from the dipole-dipole interaction of the donor and acceptor groups.

The atom types in AMBER are quite specific to amino acids and DNA bases. In the original publications, the atom types and charges are defined by means of diagrams of the amino acids and nucleotide bases.

Extensible Systematic Force Field (ESFF)

ESFF [50] was derived using a mixture of Density Functional Theory (DFT) calculations on dressed atoms to obtain polarizabilities, gas-phase and crystal structures, etc. The training set included primarily organic and organometallic compounds and a few inorganic compounds. The focus was on crystal structures and sublimation energies. The training set included models containing each element in the first 6 periods up to lead (Z = 82) (except for the inert gases), Sr, Y, Tc, La, and the lanthanides (except for Yb).

Parameters and charges are generated on the fly, based on the model configuration, the local environment, and the derived rules.

50. BIOSYM/MSI. InsightII, Release 95.0: Discover Program, versions 2.9.7 & 95.0/3.00; San Diego: Biosym/MSI, 1995.

[page 54]

The first three terms in Eq. 19 handle the internal coordinates of bonds, angles, and dihedrals. Term 3 is also used to maintain the correct chirality and tetrahedral nature of sp3 centers in the united-atom representation. (In the united-atom representation, nonpolar hydrogen atoms are not represented explicitly, but are coalesced into the description of the heavy atoms to which they are bonded.) Terms 4 and 5 account for the van der Waals and electrostatic interactions.

The final term, 6, is an optional hydrogen-bond term that augments the electrostatic description of the hydrogen bond. This term adds only about 0.5 kcal mol-1 to the hydrogen-bond energy in AMBER, so the bulk of the hydrogen-bond energy still arises

[page 55]

from the dipole–dipole interaction of the donor and acceptor groups.

The atom types in AMBER are quite specific to amino acids and DNA bases. In the original publications, the atom types and charges are defined by means of diagrams of the amino acids and nucleotide bases.

[page 36]

ESFF, extensible systematic forcefield

Derivation

ESFF was derived using a mixture of DFT calculations on dressed atoms to obtain polarizabilities, gas-phase and crystal structures, etc. The training set included primarily organic and organometallic compounds and a few inorganic compounds. The focus was on crystal structures and sublimation energies. The training set included models containing each element in the first 6 periods up to lead (Z = 82) (except for the inert gases), Sr, Y, Tc, La, and the lanthinides (except for Yb).

Parameters and charges are generated on-the-fly, based on the model configuration, the local environment, and the derived rules.

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The analytic energy expressions for the ESFF are provided in Equation (2.7). Only diagonal terms are included.

where D and K are force constants, θ are bond angles, A and B are van der Waals parameters, χ is electronegativity, q are partial charges of atoms and r are distances between atoms.

The bond energy is represented by a Morse functional form, where the bond dissociation energy D, the reference bond length r0, and the anharmonicity parameters are needed. In constructing these parameters from atomic parameters, the force field utilizes not only the atom types and bond orders, but also considers whether the bond is endo or exo to 3-, 4-, or 5-membered rings.

[page 37]

The analytic energy expressions for the ESFF forcefield are provided in Eq. 11. Only diagonal terms are included.

The bond energy is represented by a Morse functional form, where the bond dissociation energy D, the reference bond length r0 , and the anharmonicity parameters are needed. In constructing these parameters from atomic parameters, the forcefield utilizes not only the atom types and bond orders, but also considers whether the bond is endo or exo to 3-, 4-, or 5-membered rings. [...]

[page 38]

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The rules themselves depend on the electronegativity, hardness, and ionization of the atoms as well as atomic anharmonicities and the covalent radii and well depths. The latter quantities are fit parameters, and the former three are calculated.

The ESFF angle types are classified according to ring, symmetry, and π-bonding information into five groups:

• The normal class includes unconstrained angles as well as those associated with 3- , 4- and 5-membered rings. The ring angles are further classified based on whether one (exo) or both bonds (endo) are in the ring. Additionally, angles with only central atoms in a ring are also differentiated.
• The linear class includes angles with central atoms having sp hybridization, as well as angles between two axial ligands in a metal complex.
• The perpendicular class is restricted to metal centers and includes angles between axial and equatorial ligands around a metal center.
• The equatorial class includes angles between equatorial ligands of square planar (sqp), trigonal bipyramidal (tbp), octahedral (oct), pentagonal bipyramidal (pbp), and hexagonal bipyramidal (hbp) systems.
• The π−system class includes angles between pseudo atoms. This class is further differentiated in terms of normal, linear, perpendicular, and equatorial types.

The rules that determine the parameters in the functional forms depend on the ionization potential and, for equatorial angles, the periodicity. In addition to these calculated quantities, the parameters are functions of the atomic radii and well depths of the central and end atoms of the angle, and, for planar angles, two overlap quantities and the 1-3 equilibrium distances.

To avoid the discontinuities that occur in the commonly used cosine torsional potential when one of the valence angles approaches 180°, ESFF uses a functional form that includes the sine of the valence angles in the torsion. These terms ensure that the function goes smoothly to zero as either valence angle approaches 0° or 180°, as it should. The rules associated with this expression depend on the central bond order, ring size of the angles, hybridization of the atoms, and two atomic parameters for the central [atom, which is fit.]

[page 37]

The rules themselves depend on the electronegativity, hardness, and ionization of the atoms as well as atomic anharmonicities and the covalent radii and well depths. The latter quantities are fit parameters, and the former three are calculated.

[page 38]

The ESFF angle types are classified according to ring, symmetry, and π-bonding information into five groups:

• The normal class includes unconstrained angles as well as those associated with 3-, 4-, and 5-membered rings. The ring angles are further classified based on whether one (exo) or both bonds (endo) are in the ring. Additionally, angles with only central atoms in a ring are also differentiated.
• The linear class includes angles with central atoms having sp hybridization, as well as angles between two axial ligands in a metal complex.

[page 39]

• The perpendicular class is restricted to metal centers and includes angles between axial and equatorial ligands around a metal center.
• The equatorial class includes angles between equatorial ligands of square planar (sqp), trigonal bipyramidal (tbp), octahedral (oct), pentagonal bipyramidal (pbp), and hexagonal bipyramidal (hbp) systems.
• The π system class includes angles between pseudoatoms. This class is further differentiated in terms of normal, linear, perpendicular, and equatorial types.

The rules that determine the parameters in the functional forms depend on the ionization potential and, for equatorial angles, the periodicity. In addition to these calculated quantities, the parameters are functions of the atomic radii and well depths of the central and end atoms of the angle, and, for planar angles, two overlap quantities and the 1–3 equilibrium distances.

To avoid the discontinuities that occur in the commonly used cosine torsional potential when one of the valence angles approaches 180°, ESFF uses a functional form that includes the sine of the valence angles in the torsion. These terms ensure that the function goes smoothly to zero as either valence angle approaches 0° or 180°, as it should. The rules associated with this expression depend on the central bond order, ring size of the angles, hybridization of the atoms, and two atomic parameters for the central atom which is fit.

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[The rules associated with this expression depend on the central bond order, ring size of the angles, hybridization of the atoms, and two atomic parameters for the central] atom, which is fit. The functional form of the out-of-plane energy is the same as in CFF91, where the coordinate (Φ) is an average of the three possible angles associated with the out-of-plane center. The single parameter that is associated with the central atom is a fit quantity.

The charges are determined by minimizing the electrostatic energy with respect to the charges under the constraint that the sum of the charges is equal to the net charge on the molecule. This is equivalent to equalization of electronegativities.

The derivation of the rule begins with the following equation for the electrostatic energy:

$E=\sum_{i}\left ( E_i^0 + \chi _i q_i + \frac{1}{2}\eta _i q_i^2\right )+\sum_{i>j} B \frac{q_i q_j}{R_{ij}}$ (2.8)

where χ is the electronegativity and η the hardness. The first term is just a Taylor series expansion of the energy of each atom as a function of charge, and the second is the Coulomb interaction law between charges. The Coulomb law term introduces a geometry dependence that ESFF for the time being ignores, by considering only topological neighbors at effectively idealized geometries.

Minimizing the energy with respect to the charges leads to the following expression for the charge on atom i:

$q_i=\frac{\lambda - \chi_i - \Delta \chi_i }{\eta_i}$ (2.9)

where λ is the Lagrange multiplier for the constraint on the total charge, which physically is the equalized electronegativity of all the atoms. The Δχ term contains the geometry-independent remnant of the full Coulomb summation.

Equations (2.8) and (2.9) give a totally delocalized picture of the charges in a relatively severe approximation. To obtain reasonable charges as judged by, for example, crystal packing calculations, some modifications to the above picture have been made.

[page 39]

The rules associated with this expression depend on the central bond order, ring size of the angles, hybridization of the atoms, and two atomic parameters for the central atom which is fit.

The functional form of the out-of-plane energy is the same as in CFF91, where the coordinate (Φ) is an average of the three possible angles associated with the out-of-plane center. The single parameter that is associated with the central atom is a fit quantity.

The charges are determined by minimizing the electrostatic energy with respect to the charges under the constraint that the sum of the charges is equal to the net charge on the molecule. This is equivalent to equalization of electronegativities.

The derivation of the rule begins with the following equation for the electrostatic energy:

[page 40]

$E=\sum_{i}\left ( E_i^0 + \chi _i q_i + \frac{1}{2}\eta _i q_i^2\right )+\sum_{i>j} B \frac{q_i q_j}{R_{ij}}$ Eq. 12

where χ is the electronegativity and η the hardness. The first term is just a Taylor series expansion of the energy of each atom as a function of charge, and the second is the Coulomb interaction law between charges. The Coulomb law term introduces a geometry dependence that ESFF for the time being ignores, by considering only topological neighbors at effectively idealized geometries.

Minimizing the energy with respect to the charges leads to the following expression for the charge on atom i:

$q_i=\frac{\lambda - \chi_i - \Delta \chi_i }{\eta_i}$ Eq. 13

where λ is the Lagrange multiplier for the constraint on the total charge, which physically is the equalized electronegativity of all the atoms. The ∆χ term contains the geometry-independent remnant of the full Coulomb summation.

Eqns. 12 and 13 give a totally delocalized picture of the charges in a relatively severe approximation. To obtain reasonable charges as judged by, for example, crystal packing calculations, some modifications to the above picture have been made.

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Metals and their immediate ligands are treated with the above prescription, summing their formal charges to get a net fragment charge. Delocalized π−systems are treated in an analogous fashion. σ−systems are treated using a localized approach in which the charges of an atom depend simply on its neighbors. Note that this approach, unlike the straightforward implementations based on the equalization of electronegativity, does include some resonance effects in the π−system.

The electronegativity and hardness in the above equations must be determined. In earlier force fields they were often determined from experimental ionization potentials and electron affinities; however, these spectroscopic states do not correspond to the valence states involved in molecules. For this reason, ESFF is based on electronegativities and hardnesses, calculated using density functional theory. The orbitals are (fractionally) occupied in ratios appropriate for the desired hybridization state, and calculations are performed on the neutral atom as well as on positive and negative ions.

ESFF uses the 6-9 potential for the van der Waals interactions. Since the van der Waals parameters must be consistent with the charges, they are derived using rules that are consistent with the charges.

Starting with the London formula:

$\left ( B_i \sim \alpha_i^2.\mathit{IP} \right )$ (2.10)

where α is the polarizability and IP the ionization potential of the atoms, the polarizability, in a simple harmonic approximation, is proportional to n/IP where n is the number of electrons. Across any one row of the periodic table, the core electrons remain unchanged, so that the following form is reasonable:

$\alpha=\frac{a'}{\mathit{IP}}+\frac{b' n_{\mathit{eff}}}{\mathit{IP}}$ (2.11)

[page 40]

Metals and their immediate ligands are treated with the above prescription, summing their formal charges to get a net fragment charge. Delocalized π systems are treated in an analogous fashion. And σ systems are treated using a localized approach in which the charges of an atom depend simply on its neighbors. Note that this approach, unlike the straightforward implementations based on the equalization of electronegativity, does include some resonance effects in the π system.

Electronegativity and hardness obtained by DFT

The electronegativity and hardness in the above equations must be determined. In earlier forcefields they were often determined from experimental ionization potentials and electron affinities; however, these spectroscopic states do not correspond to the valence states involved in molecules. For this reason, ESFF is based on electronegativities and hardnesses, calculated using density functional theory as implemented in DMol. The orbitals are (fraction-

[page 41]

ally) occupied in ratios appropriate for the desired hybridization state, and calculations are performed on the neutral atom as well as on positive and negative ions.

van der Waals interactions

ESFF uses the 6–9 potential for the van der Waals interactions. Since the van der Waals parameters must be consistent with the charges, they are derived using rules that are consistent with the charges.

Derivations

Starting with the London formula:

$( B_i \sim \alpha_i^2 \cdot \mathit{IP} )$ Eq. 14

where α is the polarizability and IP the ionization potential of the atoms, the polarizability, in a simple harmonic approximation, is proportional to n / IP where n is the number of electrons. Across any one row of the periodic table, the core electrons remain unchanged, so that the following form is reasonable:

$\alpha=\frac{a'}{\mathit{IP}}+\frac{b' n_{\mathrm{eff}}}{\mathit{IP}}$ Eq. 15

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where and are adjustable parameters that should depend on just the period, and neff is the effective number of (valence) electrons. Further assuming that α is proportional to R3 and that another equivalent expression to that in Equation (2.10) is:

$B_i \sim \varepsilon R^6$ (2.12)

where ε is a well depth, the following forms are deduced for the rules for van der Waals parameters:

$R_i=\frac{a}{(\mathit{IP})^{1/3}}+ \frac{b.n_{\mathit{eff}}^{1/3}}{(\mathit{IP})^{1/3}} \quad \textrm{and } \qquad \varepsilon_i=c(\mathit{IP})$ (2.13)

The van der Waals parameters are affected by the charge of the atom.

In ESFF we found it sufficient to modify the ionization potential (IP) of metal atoms according to their formal charge and hardness:

$\mathit{IP} = (\mathit{IP})_0 + q\eta _i$ (2.14)

and for nonmetals to account for the partial charges when calculating the effective number of electrons.

ESFF atom types are determined by hybridization, formal charge, and symmetry rules. In addition, the rules may involve bond order, ring size, and whether bonds are endo or exo to rings. For metal ligands the cis-trans and axial-equatorial positionings are also considered. The additions of these latter types affect only certain parameters (for example, bond order influences only bond parameters) and thus are not as powerful as complete atom types. In one sense they provide a further refinement of typing beyond atom types.

The ESFF has been parameterized to handle all elements in the periodic table up to radon. It is recommended for organometallic systems and other systems for which other [force fields do not have parameters.]

[page 41]

where a´ and b´ are adjustable parameters that should depend on just the period, and neff is the effective number of (valence) electrons. Further assuming that α is proportional to R3 and that another equivalent expression to that in Eq. 14 is:

$B_i \sim \varepsilon R^6$ Eq. 16

where ε is a well depth, the following forms are deduced for the rules for van der Waals parameters:

$R_i=\frac{a}{(\mathit{IP})^{1/3}}+ \frac{b \cdot n_{\mathrm{eff}}^{1/3}}{(\mathit{IP})^{1/3}} \quad \textrm{and } \qquad \varepsilon_i=c(\mathit{IP})$ Eq. 17

The van der Waals parameters are affected by the charge of the atom.

In ESFF we found it sufficient to modify the ionization potential (IP) of metal atoms according to their formal charge and hardness:

$\mathit{IP} = (\mathit{IP})_0 + q\eta _i$ Eq. 18

[page 42]

and for nonmetals to account for the partial charges when calculating the effective number of electrons.

ESFF atom types (Table 32) are determined by hybridization, formal charge, and symmetry rules (Atom-typing rules in ESFF). In addition, the rules may involve bond order, ring size, and whether bonds are endo or exo to rings. For metal ligands the cis–trans and axial–equatorial positionings are also considered. The addition of these latter types affects only certain parameters (for example, bond order influences only bond parameters) and thus are not as powerful as complete atom types. In one sense they provide a further refinement of typing beyond atom types.

The ESFF forcefield has been parameterized to handle all elements in the periodic table up to radon. It is recommended for organometallic systems and other systems for which other forcefields do not have parameters.

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Quelle: Accelrys Inc. - Forcefield-Based Simulations 1998
Seite(n): 42, Zeilen: 13-21
[It is recommended for organometallic systems and other systems for which other] force fields do not have parameters. ESFF is designed primarily for predicting reasonable structures (both intra- and intermolecular structures and crystals) and should give reasonable structures for organic, biological, organometallic and some ceramic and silicate models. It has been used with some success for studying interactions of molecules with metal surfaces. Predicted intermolecular binding energies should be considered approximate. It is recommended for organometallic systems and other systems for which other forcefields do not have parameters. ESFF is designed primarily for predicting reasonable structures (both intra- and intermolecular structures and crystals) and should give reasonable structures for organic, biological, organometallic and some ceramic and silicate models. It has been used with some success for studying interactions of molecules with metal surfaces. Predicted intermolecular binding energies should be considered approximate.
 Anmerkungen No source is given. Sichter (Klgn), WiseWoman