The nonlinear correlation factors included in F12 wavefunctions are of the form exp (-γr 12 ), where γ is the geminal Slater exponent (see Ref. and references therein). The optimal value of γ depends on the chemical system, the orbital basis set, and the electrons being correlated. For example, a γ value of 1.4 a -1 0 has been found to work well for the core electron correlation effects on lighter p-block elements, but values closer to 1.0 a -1 0 produce energetically favourable results for valence-only correlation. For some elements, particularly those further down the periodic table, appropriate basis sets (orbital and/or auxiliary) and γ values for the calculation of energies and/or properties with F12 methods have not yet been established.