| CFOUR (Coupled-Cluster techniques for Computational Chemistry) is a program package for performing high-level quantum mechanical 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 offer decided advantages when configuration mixing is important. At present, these include the EOMEE approach for singlet and triplet excited states of a closed-shell molecule, 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 fifteen years. 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. |
