An OpenMP RI-MP2 energy algorithm is used by default in Q-Chem 4.1 onward. This can be invoked by using CORR = PRIMP2 for older versions, but note that in 4.01 and below, only RHF/RI-MP2 was supported. Now UHF/RI-MP2 and ROHF/RI-MP2 is supported, and the formation of the ‘B’ matrices as well as three center integrals are parallelized. This algorithm uses the remaining memory from the MEM_TOTAL allocation for all computation, which can drastically reduce hard drive reads in the formation of t-amplitudes. Since Q-Chem 5.2, RI-MP2 can be invoked by using method=mp2 and proper auxiliary basis using AUX_BASIS_CORR. In case of double-hybrid, adding AUX_BASIS_CORR will simply invoke RI-MP2.
$molecule 0 1 C1 H1 C1 1.077260 H2 C1 1.077260 H1 131.608240 $end $rem EXCHANGE HF CORRELATION pRIMP2 BASIS cc-pVTZ AUX_BASIS rimp2-cc-pVTZ PURECART 11111 SYMMETRY false THRESH 12 SCF_CONVERGENCE 8 MAX_SUB_FILE_NUM 128 !TIME_MP2 true $end