The self-consistent electron pairs method for multiconfiguration reference state functions

An efficient direct CI method which includes all singly and doubly substituted configurations with respect to an arbitrary multiconfiguration (MCSCF) reference function is described. The configurations are generated by subsequently applying spin-coupled two-particle annihilation and creation operators to the complete MCSCF function. This considerably reduces the size of the n-electron basis and the computational effort as compared to previous multireference CI treatments, in which the configurations are defined with respect to the individual reference configurations. The formalism of the method is very similar to the closed-shell ’’self-consistent electron pairs’’ (SCEP) method of Meyer. The vector Hc is obtained in terms of simple matrix operations involving coefficient and integral matrices. A full transformation of the two-electron integrals is not required. Test calculations with large basis sets have been performed for the 3B1 and 1A1 states of CH2 (ΔE = 9.5 kcal/mol) and for the CH2(3B1) +H2→CH3+H reaction barrier (ΔE = 10.7 kcal/mol). As a preliminary test for the accuracy of the results obtained with contracted wave functions of the above type the potential energy and dipole moment functions of the OH X 2Π and A 2Σ+ states have been calculated. For the 2Π state re and ωe deviate by less than 10−3 Å and 1 cm−1, respectively, from the experimental data. For the 2Σ+ state the agreement is somewhat less good, which is probably due to basis set defects. Around the equilibrium distance the calculated dipole moment functions are in very close agreement with those previously obtained from PNO– CEPA functions.