Three- and four-body nonadditivities in nucleic acid tetramers: a CCSD(T) study.

Three- and four-body nonadditivities in the uracil tetramer (in DNA-like geometry) and the GC step (in crystal geometry) were investigated at various levels of the wave-function theory: HF, MP2, MP3, L-CCD, CCSD and CCSD(T). All of the calculations were performed using the 6-31G**(0.25,0.15) basis set, whereas the HF, MP2 and the MP3 nonadditivities were, for the sake of comparison, also determined with the much larger aug-cc-pVDZ basis set. The HF and MP2 levels do not provide reliable values for many-body terms, making it necessary to go beyond the MP2 level. The benchmark CCSD(T) three- and four-body nonadditivities are reasonably well reproduced at the MP3 level, and almost quantitative agreement is obtained (fortuitously) either on the L-CCD level or as an average of the MP3 and the CCSD results. Reliable values of many-body terms (especially their higher-order correlation contributions) are obtained already when the rather small 6-31G**(0.25,0.15) basis set is used. The four-body term is much smaller when compared to the three-body terms, but it is definitely not negligible, e.g. in the case of the GC step it represents about 16% of all of the three- and four-body terms. While investigating the geometry dependence of many-body terms for the GG step at the MP3/6-31G**(0.25,0.15) level, we found that it is necessary to include at least three-body terms in the determination of optimal geometry parameters.