C 6 dispersion coefficients in multiconfiguration self-consistent field linear response theory

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Abstract
An algorithm for the evaluation of correlated dipole–dipole dispersion coefficients by direct MCSCF linear response theory is presented. Stepwise construction of a pseudo‐state basis using eigenvectors of successive (Cauchy) moments of the linear response function gives an efficient scheme for obtaining polarizabilities at imaginary frequency. The scheme is also useful for polarizabilities on the real axis below the frequency of the first dipole‐allowed transition. Sample calculations of C6 coefficients are described for a range of two‐electron (H−, He, Li+, Be2+, H2, and H+3 ) and many‐electron (Be, N, N2 ) closed‐ and open‐shell systems.