A COMPARISON BETWEEN THE NEMO INTERMOLECULAR WATER POTENTIAL AND AB-INITIO QUANTUM-CHEMICAL CALCULATIONS FOR THE WATER TRIMER AND TETRAMER

A comparison between recent ab initio quantum chemical calculations an d model potential results for an intermolecular water potential is mad e for different stationary points in the low energy region of the pote ntial energy surface of the cyclic homodromic water trimer and tetrame r. Relative energies, structures and vibrational frequencies are compa red for the local stationary points of the potential energy surfaces. The intermolecular model potential was derived almost exclusively from water monomer properties, by use of the NEMO method. The comparison s hows good agreement between the NEMO water potential and MP2-R12 calcu lations. The use of accurate intermolecular potentials instead of ab i nitio methods allows for the treatment of much larger chemical system due to extensive savings in CPU time. A single point MP2-R12 energy ca lculation for a non-symmetrical water tetramer conformer without symme try takes about 40 CPU hours compared with less than 0.5 second for th e most sophisticated Version of the NEMO potential.