The van der Waals function is given by

where **H** is the heavy-side function and **SW** is a
switching function. **SW** has the form

For both the truncated and the switched option,
the van der Waals function is described by a
Lennard-Jones potential.
In this potential, the attractive
force is proportional to , while the repulsive force
varies as .
**A**,**B** and , are related to the well depth
and the minimum distance (van der Waals radius) by

The repel option uses a simple repulsive potential. It is used primarily for structure refinements with X-ray crystallographic and solution NMR spectroscopic data.

The NBON statement (Section 3.2.1) defines for the Lennard-Jones potential between identical atom types. Between different atom types, the following combination rule is used by default:

The NBFix statement allows
one to deviate from this combination rule; i.e,
one can explicitly specify the **A,B** coefficients for an atom type pair
(see Section 3.2.1).

Minimization of the empirical potential energy for initial coordinates with very close nonbonded contacts is often ill behaved because the Lennard-Jones potential produces a very large gradient reflecting the close contacts. To avoid this problem, the Lennard-Jones and electrostatic potential can be replaced with the repel potential, which is softer and purely repulsive.

Sat Mar 11 09:37:37 PST 1995