Modeling of the exchange repulsion energy

The nonempirical force field model (NEMO) has been augmented with a new exchange repulsion model based on a different choice of expansion center. In the new repulsion model, the size of the atoms are estimated from the trace of the local second-order electron density tensor divided with the local valence charge. A set of general atom type dependent parameters are fitted from SCF calculations on different complexes. An exponential form is used to describe the interaction. The new expansion center is chosen as the point where the local electronic dipole is zero, i.e. the center of electronic charge. The model has been tested on a set of intermolecular complexes between the molecules water, ammonia, methylamine, formamide, and urea.