Alternative formulation of the matrix elements in MP2-R12 theory

A formulation of MP2-R12 theory is presented leading to simplified matrix elements in the R12 part. The resulting formulas are free from the integrals over the commutator of the kinetic operator with inter-electronic distance, which means a substantial decreasing of the computational demands. The use of an auxiliary basis set for the resolution of identity is discussed. First results using the standard approximation are compared with the former variants of MP2-R12 for CH2, CH4, NH3, H2O, HF, CO, CH,OH, N-2, and F-2. The results show that the new formulation is not only less demanding, but also preferred concerning the convergence properties toward the basis set limit. (c) 2005 Wiley Periodicals, Inc.