Intermolecular Interaction, Mp2,-of R12 Theory

Beyond the Hartree-Fock level, the slow convergence problem becomes the bottleneck for computing highly accurate molecular electronic energies and properties. Experience has shown, for example, that the computation time of a correlated electronic-structure calculation in a correlation-consistent cc-pVXZ basis grows as∝X¹² with its cardinal number X, while basis-set truncation errors only disappear as∝X^-3. The slow convergence reason is the computation of integral, especially the multicenter integrals.In 1985, the linear R12 methods were introduced by Kutzelnigg and co-workers. These methods employ the so-called "RI" by which one can avoid evaluation of expensive multicenter integrals, only the two-electron integrals. Such integrals can be computed rather efficiently. In 1987, W.Klopper and W.Kutzelnigg introduced the second-order Moller-Plesset linear-r₁₂ (MP2-R12) depended on the linear R12 methods.After the MP2-R12 method was introduced, in practice, it not only will reach reliable results, but also superiority of efficiency. However it is a method based on the single-molecule system. For it is used on the molecule interaction, nobody hasn't done the special work. This paper will discuss that the explicitly MP2-R12 method was used on molecule interaction about its feasibility,accuracy and computation efficiency.To examine the size consistency of the MP2-R12 method, we set 15 for the integral tolerance and 10 for the SCF convergence in the computation program, and compute some of the representative systems. The results shown that the 8 value is very small; the P value is very small no more than 10 millionths. Compared with MP2 method, theδvalue and the P value are small.To calculate binding energy, first we obtain the criterion value△Eref based on CCSD (T) /aug-cc-pVTZ//MP2/aug-cc-pVTZ level through BSSE correction. the BSSE correction is Counterpoise Procedure correction. For the computation systems, including the weak interaction systems,the typical hydrogen bonding interaction systems,strong hydrogen bonding interaction systems and unconventional hydrogen bonding interaction systems such as X－H…p systems, we obtain the binding energy in MP2-R12/aug-cc-pVTZ//MP2/aug-cc-pVDZ level after using CP correction for BSSE, the results showed: their errors compare with their△Eref is about 2 kJ/mol, the largest error does not exceed 4kJ/mol.