CFOUR (Coupled-Cluster techniques for Computational Chemistry) is a program package for performing high-level quantum chemical calculations on atoms and molecules. The major strength of the program suite is its rather sophisticated arsenal of high-level ab initio methods for the calculation of atomic and molecular properties. Virtually all approaches based on Møller-Plesset (MP) perturbation theory and the coupled-cluster approximation (CC) are available; most of these have complementary analytic derivative approaches within the package as well. Studies of excited electronic states and other ”multireference” problems are possible using the equation-of-motion (EOM) coupled-cluster techniques. These techniques which are closely related to (and in some cases identical to) so-called Fock space multireference coupled-cluster theory, offer a powerful means to study open-shell systems and decided advantages when configuration mixing is important. At present, these include the EOMEE approach for singlet and triplet excited states, and the EOMIP and EOMEA methods that are best applied to low-spin doublet states. Analytic derivatives are available for these methods. A number of methodological developments have been added to the program in the last two decades. These include: analytic second derivatives for all coupled-cluster approaches up to full CCSDT, the calculation of NMR chemical shifts at MP and CC levels of theory, the calculation of anharmonic force fields (via numerical differentation of analytic derivatives), relativistic corrections, corrections to the Born-Oppenheimer approximation at the CC level, nonadiabatic coupling within the EOM framework, and several others
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