where F C (C = A or B) denotes Fock operator describing the monomer C, W C is the correlation operator and V AB is the intermolecular interaction operator. The simplest way of calculating the dispersion energy can be obtained when λ A = λ B = 0. In this case, the dispersion energy interprets as the correlation energy of two monomers described by Hartree-Fock wavefunctions only. Interestingly such dispersion energy is recovered by MP2 supermolecular calculations. In further works of Szalewicz and Jeziorski, and Rybak et al. higher order corrections to the intramonomer correlation effects in the dispersion energy were introduced. This approach was later improved by infinite-order summation techniques, based on random-phase (ring) approximation 14 or coupled-cluster method. It turned out that for some van der Waals complexes, the infinite-order summation can be very important, eg. for the molecules with multiple bonds. An alternative way for evaluating the dispersion energy is to take advantage of the formula proposed by Longuet-Higgins, in which the dispersion energy is expressed in terms of a product of polarization propagators integrated over the imaginary frequencies and contracted with coulomb integrals. This methodology was initiated by Jaszuński and McWeeny 18 by employing the time-dependent Hartree-Fock (TDHF) response function. Such an approach is very general and can be applied with any kind of linear response function. In particular timedependent density functional theory (TDDFT) was used to obtain the dispersion energy in the framework of DFT which opened opportunity for calculations of accurate interaction energies for large systems in the DFT framework. Korona and Jeziorski 21 demonstrated that such an approach can work very effectively with time-independent coupled cluster response function introduced by Moszynski et al. at the CCSD level of accuracy for monomer. More recently the dispersion was obtained in a similar manner from extended-RPA (EERPA) theory by Hapka et. al which can be combined with any method providing 2-particle reduced density matrices (2-RDMs).