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# 6.6.3 OpenMP Implementation of RI-MP2

(February 4, 2022)

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.

Example 6.4  Example of OpenMP-parallel RI-MP2 job.

$molecule 0 1 C1 H1 C1 1.077260 H2 C1 1.077260 H1 131.608240$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


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