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4.3 Method and wavefunction specification

After defining the geometry and basis set (in any order) one has to specify the methods to be used. This is simply done by keywords, which are normally the same as the usual abbreviations for the methods (HF for Hartree-Fock, MP2 for second-order Møller-Plesset perturbation theory, or CCSD(T) for coupled-cluster with singles and doubles and perturbative triples. In most cases, the first step is a Hartree-Fock calculation, in which the molecular orbitals to be used in subsequent electron correlation treatments are optimized. An arbitary number of different methods can be executed after each other, just by giving the corresponding keywords. For example

hf        !orbital optimization using HF
mp2       !MP2 calculation using the HF orbitals
mp4       !MP4 calculation using the HF orbitals
ccsd(t)   !CCSD(T) calculation using the HF orbitals

Note, howeverm that MP2 is part of MP4 and CCSD, and therefore the mp2 calculation in the above input is redundant. 2017-12-11