The extrapolation functional is chosen by a keyword with the METHOD, METHOD_R, and/or METHOD_C options. The default functional is L3. In the following, is the cardinal number of the basis set (e.g., 2 for VDZ, 3 for VTZ etc), and is an arbitrary number. is a constant given either by the PR or PC options (default ). X is a number or a vector given either by the XR or XC options (only for LX; is the number of elements provided in X). , , are the fitting coefficients that are optioized by least-squares fits.
The following example shows various possibilities for extrapolation:
The second example shows extrapolations of MRCI energies. In this case both the MRCI and the MRCI+Q energies are extrapolated.