Hello,

I encountered the following, at first glance counter-intuitive result for an rimp2 calculation for a deprotonated (anionic) closed-shell molecule.

Since it is anion diffuse functions could be important. So, I set up a mp2 calculation using the aug-cc-pVDZ basis set.

For some reason I changed only the main basis to aug-cc-pVDZ but the auxiliary basis (cbas), which was set to cc-pVDZ. This is case 1.

When I saw the inconsistency, I repeated the calculation with the right, i.e. aug-cc-pVDZ, auxiliary basis set. This is case 2.

When I compared the mp2 energies in case 1 and 2 I found out that the mp2 energy in case 1 (inconsistent cbas basis set) is slightly

(~0.2 kcal/mol) lower than the energy in case 2 (the main and cbas basis sets are consistent). This was unexpected as I thought that

the larger the cbas basis set, the more closer the result to that obtained without the use of RI. In fact it turned out that the mp2 energy

obtained with the smaller cbas basis set, i.e. case 1, is slightly lower than the mp2 energy obtained without the use of RI.

I wounder whether the above described queerness is known and what the reason of it is (can be)?

Best regards,

Evgeniy