Universal Basis Sets and Direct ccMBPT

2 Universal Basis Sets and Direct ccMBPT. - Early many-body perturbation theory calculations carried out within the algebraic approximation quickly led to the realization that basis set truncation is the dominant source of error in correlation studies seeking high precision when carried out with respect to an apprpriately chosen reference function. In more recent years, the importance of basis set truncation error control has been more widely recognized. We have described the concept of the universal basis set in Section 2.4.4 which provides a general approach to basis set truncation error reduction. 

The development of ccMBPT algorithms, which, as described in Section 2.7, effectively exploit the power of parallel processing machines, and direct techniques,39-42 which avoid the need to store the many two-electron integrals arising, has facilitated the use of the large and flexible basis sets of the universal type. 

In Table 16, the second order electron correlation energies reported by Moncrieff and Wilson173 using the basis sets defined in Table 15 are compared with the second order correlation energies obtained with a large correlation consistent basis set, the aug-cc-pCV5Z set, which support a description of both valence and core correlation effects. 

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