Automatic Generation of Flexible-Monomer Intermolecular Potential Energy Surfaces

A method is developed
for automatic generation of nonreactive intermolecular
two-body potential energy surfaces (PESs) including intramonomer degrees
of freedom. This method, called
flex-autoPES, is an extension of the autoPES method developed earlier,
which assumes rigid monomers. In both cases, the whole PES development
proceeds without any human intervention. The functional form used
is a sum of products of site–site functions (both atomic and
off-atomic sites can be used). The leading terms with sites involving
different monomers are of physically motivated form. The long-range
part of a PES is computed from monomer properties without using any
dimer information. The close-range part is fitted to dimer interaction
energies computed using electronic structure methods. Virtually any
method can be used in such calculations, but the use of symmetry-adapted
perturbation theory provides a seamless connection to the long-range
part of the PES. The performance of the flex-autoPES code was tested
by developing a full-dimensional PES for the water dimer and PESs
including only some soft intramonomer degrees of freedom for the ethylene
glycol dimer and for the ethylene glycol–water dimer. In the
case of the water dimer, the root-mean-square error (RMSE) of the
PES from the data points with negative total energies is 0.03 kcal/mol,
and we expect this PES to be more accurate than any previously published
PES of this type. For the ethylene glycol dimer and the ethylene glycol–water
dimers, the analogous RMSEs are 0.25 and 0.1 kcal/mol, respectively.