Development of anharmonic vibrational state theory based on quantum Monte Carlo method and applications to the binding of a positron to non-polar molecule

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In order to elucidate the mechanism of the binding of a positron to non-polar molecules, we have developed a new anharmonic vibrational state theory based on quantum Monte Carlo method, and have applied our theory together with ab initio multi-component molecular orbital theory to non-polar carbon disulfide (CS2) molecule as well as some polar molecules such as hydrogen cyanide (HCN) and formaldehyde (H2CO) molecules. Analyzing vibrational averaged positron affinities, which is the binding energy of a positron to molecules including the effect of molecular vibration, we found that the vibrational excitations of the CH stretching modes for HCN molecule, CO stretching modes for H2CO molecule, and anti-symmetric stretching modes for CS2 molecule enhance the PA value compared to that of the vibrational ground state. We confirmed that for all molecules the PA variation arises from the variation of permanent dipole moment and dipole-polarizability at each vibrational state.