The inclusion of electron correlation in intermolecular potentials: applications to the formamide dimer and liquid formamide

A test of the quality of the electrostatic properties and polarizabilities used in the nonempirical molecular orbital (NEMO) potential is carried out for formamide by calculating the molecular dipole moment and polarizability at the second-order Moller-Plesset (MP2) level of theory. The molecular di pole moment is 11% lower at the MP2 level than at the Hartree-Fock (HF) lev el, whereas the isotropic part of the polarizability is increased by 36% by adding electron correlation and using a considerably larger basis set. The atomic charges, dipole moments and polarizabilities obtained at the HF lev el are rescaled to get the correct molecular properties at the MP2 level. T he potential minimum for the cyclic dimer of formamide is -17.50 kcal/mol w ith the MP2-scaled properties and is significantly lower than other potenti als give. Two intermolecular potentials are constructed and used in subsequ ent molecular dynamics simulations: one with the regular NEMO potential and the other with the rescaled MP2 properties. A damping of the electrostatic field at short intermolecular distances is included in the present NEMO mo del. The average energies for liquid formamide are lower for the MP2-scaled model and are in good agreement with experimental results. The lowering of the simulation energy for the MP2-scaled potential indicates the strong di spersive interactions in liquid formamide.