Subject: CCL:summary of responses about esff

From: [email-masked] (Shenghua Shi )
Date: Wed, 9 Aug 1995 16:22:18 -0700

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>Subject: CCL:summary of responses about esff
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>hi,
>>This  is the  summary   of  answers  i got   about the
>esff  force field.
>
>

ESFF is an Extensible and Systematic Forcefield developed at Biosym 
for molecular modeling of organic, inorganic, and organometallic systems. 
It covers all the elements in the periodic table up to Rn.  Currently, ESFF 
is able to predict structures of isolated molecules as well as systems of 
condensed phases.  In addition, it can be used to predict properties related 
to non-bonded interactions, such as sublimation energies, with reasonable 
accuracy.  

In the upcoming Discover release, systems treated by ESFF can have up to 10000 
atoms per unit cell.

Thanks to input from many of our customers, ESFF has improved considerably 
since its first (beta) version which was released a year ago.  The preliminary 
results of a number of in-house and customer applications indicate that ESFF 
reproduces experimental structures fairly well for a variety of systems 
including nucleic acids, porphyrins, transition metal complexes,  and 
organometallic compounds. 

The following is a brief summary of the ESFF methodology:
   
ESFF is a diagonal valence forcefield.  The valence bond, angle, torsion,
and out-of-plane energies are used to describe the internal interactions,
while Van Der Waals (VDW) and electrostatic energies represent the non-bonded 
interactions. 

In many systems, specifically metal complexes, deviations in bond lengths and
bond angles from equilibrium geometries are large.  To account for this fact 
and the periodicity of angle energies in coordination compounds, we have 
implemented Morse functions for bond energies and truncated cosine series for 
angles energies. 

A novel functional form is used to describe the torsion energies. It is 
essentially an entire function of dot products of unit vectors.  This type of 
function insures that there is no singularity when the bond angles approach 180 
as well as significantly simplifies the calculation of the Cartesian derivatives
of the torsion energies.  Moreover, the parameters in the VDW energy terms, 
like the electrostatic term, are products of single atomic quantities.
The advantage of such expressions is that the nonbond energy can be computed 
more efficiently using advanced techniques such as the Cell Multipole method or 
methodologies based on Fast Fourier Transform.

The forcefield parameters are generated from atomic parameters based on rules 
that embody the physics of atom-atom interactions. The number of atomic 
parameters scale as a linear function of the total number of atom types.
Although, this approach has been used in the past, the rules have been too 
general to yield a forcefield with sufficient accuracy.  We have extended the 
philosophy of the approach in several ways to improve the accuracy and 
parameter transferability:

1) We have augmented the specificity of atom types with the inclusion of
internal type classification to capture detailed behaviors associated with
particular internals. The atomic parameters depend not only on the atom types 
but also the internal types. This allows for a more accurate representation of 
constrained systems, conjugated systems, dative effect (due to sigma lone pair 
- vacant orbital interaction), pi dative effect (resulting from pi lone 
pair-vacant orbital interaction), electron back donating effect, and 
Jahn-Teller effect.

2) By introducing a concept of pseudo-angles, the ESFF forcefield can 
reproduce unconventional structures resulting from d-p pi bonding between
the unsaturated ligands and transition metals.

3) Many of the rules used in ESFF have been derived from approximate models 
which grasp the essense of the underlying physics. These rules have enabled us 
to produce a relatively small set of physically meaningful and transferable 
parameters.  For example, by combining London's formula for dispersion energy 
with a harmonic oscillator model for the polarization, semiempirical expressions
for the VDW radii and well depth have been derived.  The expressions represent 
such a good correlation that more than a thousand VDW parameters for all atom 
types are generated from as few as ~30 atomic parameters.

4) Considering the complexity of the physics involved in atomic interactions, 
we have derived schemes to use a series of rules instead of a single rule in 
order to account for various critical effects. For example, the partial atomic 
charges are calculated using an elaborated scheme whereby atoms are grouped 
into different charge groups according to their bonding nature. Depending on 
the group, the partial charges are determined using different algorithms 
derived from either the group electronegativity equalization or the local 
electronegativity equalization (for a pair of bonded atoms). The scheme also 
takes into account the inductive, resonance and field effects in the different 
stages of calculation.

5) We have developed a unique algorithm to generate the fundamental atomic 
parameters, such as electronegativity, hardness and ionization potential
for all atom types (ie. atoms having different hybridizations, oxidation 
numbers, coordination numbers or symmetries) by making use of the density 
functional theory.

A more detailed description of ESFF parameterization is available on request.

The development and enchancement of the ESFF forcefield is a continuous effort 
at Biosym.  Therefore, we gladly welcome feedback from our users. Please do not
hesitate to contact our response center or me if you have any questions or
comments regarding ESFF.
         
                                   Thanks,
                     
                                                  shenghua
++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Shenghua Shi
BIOSYM Technologies
[email-masked]
619-546-5348