# 6.6.6 RI-TRIM MP2 Energies

The triatomics in molecules (TRIM) local correlation approximation to MP2 theory561 was described in detail in Section 6.5.1 which also discussed our implementation of this approach based on conventional four-center two-electron integrals. Starting from Q-Chem v. 3.0, an auxiliary basis implementation of the TRIM model is available. The new RI-TRIM MP2 energy algorithm222 greatly accelerates these local correlation calculations (often by an order of magnitude or more for the correlation part), which scale with the 4th power of molecule size. The electron correlation part of the calculation is speeded up over normal RI-MP2 by a factor proportional to the number of atoms in the molecule. For a hexadecapeptide, for instance, the speedup is approximately a factor of 4.222 The TRIM model can also be applied to the scaled opposite spin models discussed above. As for the other RI-based models discussed in this section, we recommend using RI-TRIM MP2 instead of the conventional TRIM MP2 code whenever run-time of the job is a significant issue. As for RI-MP2 itself, TRIM MP2 is invoked by adding AUX_BASIS $rems to the input deck, in addition to requesting CORRELATION = RILMP2. Example 6.15 Example of RI-TRIM MP2 energy evaluation $molecule
0 3
C1
H1   C1   1.07726
H2   C1   1.07726   H1   131.60824
$end$rem
METHOD         rilmp2
BASIS          cc-pVDZ
AUX_BASIS      rimp2-cc-pVDZ
PURECART       1111
UNRESTRICTED   true
SYMMETRY       false
\$end