Explicitly correlated coupled cluster methods with pair-specific geminals

Explicitly correlated MP2-F12 and CCSD(T)-F12 methods with orbital-pair-specific Slater-type geminals are proposed. The fixed amplitude ansatz of Ten-no is used, and different exponents of the Slater geminal functions can be chosen for core-core, core-valence, and valence-valence pairs. This takes care of the different sizes of the correlation hole and leads to improved results when inner-shell orbitals are correlated. The complications and the extra computational cost as compared to corresponding calculations with a single geminal are minor. The improved accuracy of the method is demonstrated for spectroscopic properties of Br(2), As(2), Ga(2), Cu(2), GaCl, CuCl, and CuBr, where the d-orbitals are treated as core.