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 spinhyphen;coupled twohyphen;particle annihilation and creation operators to the complete MCSCF function. This considerably reduces the size of thenhyphen;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 closedhyphen;shell rsquo;rsquo;selfhyphen;consistent electron pairsrsquo;rsquo; (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 twohyphen;electron integrals is not required. Test calculations with large basis sets have been performed for the3B1and1A1states of CH2(Dgr;E= 9.5 kcal/mol) and for the CH2(3B1) +H2rarr;CH3+H reaction barrier (Dgr;E= 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 OHXthinsp;2Pgr; andAthinsp;2Sgr;+states have been calculated. For the2Pgr; statereand ohgr;edeviate by less than 10minus;3Aring; and 1 cmminus;1, respectively, from the experimental data. For the2Sgr;+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 PNOndash; CEPA functions.
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