Effective core potential and Gaussian basis set Is using the effective core potential (ECP) and Gaussian basis set simultaneously in ab initio calculation a valid approach? For example, is it correct to use ECP for one of the atoms and, instead of a basis set which is optimized for ECP, to take a normal Gaussian basis set (say, 6-31G) when we perform calculations for a molecule with a Hartree–Fock method?
P.S. I know technically I can do it in the quantum chemistry packages (for example, I have checked it in MOLPRO), but I don't know whether it is valid physically.
Sure, it can be valid physically. You can use the full-electron basis set, like 6-31G*, and the ECP with associated basis set, like Lanl2dz, for instance, in calculations of transition metal compounds or compounds with heavier non-metals (Sb, Te, etc.) as an example.
Important here would be to compare results obtained with available experimental data (such as bond distances/angles, reaction energies, UV-vis spectra, etc.) or results of similar calculations in order to see if the simultaneous use of ECP and full-electron basis sets is valid and would not give you some weird results.