Towards the elaboration of a QM method to describe molecular solutes under the effect of a very high pressure

In this paper, we present a new computational QM method for the study of structural properties (i.e. equilibrium geometry) of molecular systems at very high pressure. The procedure is based on the polarizable continuum model (PCM) method, usually used to study molecular solutes under standard pressure conditions. The paper considers two critical items: the definition of the pressure and the elaboration of an analytical code for the calculation of molecular gradients. The method has been developed at HF and DFT levels, with computational costs comparable with those of similar calculations in vacuo. The numerical examples regarding the equilibrium geometries and conformational energies of 1,3-butadiene under high pressure give an indication of the potentialities of the approach and of the problems to which it may be applied.