RPBE functional

A quasichemical theory, using ab initio simulation (rPBE functional) to generate data for the computation of the various contributions to the free energy, yields an estimate for the excess chemical potential of water very close to the experimental value. 

There is an interesting exception to the rule that pure functionals predict near-degenerate singlet and triplet states. The modified form of the pure PBE functional described by Hammer et al [14], called RPBE, has been reported [35] to predict a triplet-singlet energy difference for Fe(CO)4 of 4.0 kcal/mol, which is probably much closer to the correct value than that computed with any other pure functional. Whilst the RPBE functional has been very enthusiastically adopted by the periodic DFT community for calculations on surfaces - the area for which it was developed - very few applications have been reported for molecular TM complexes, so it is not yet clear whether the good result for Fe(CO)4 will be transposable to other systems. 

Preliminary cluster studies [129] on the electronic structure of the trigonal V203(0001) surface substantiate the above concepts. As an example. Table 3, lists atom charges from Mulliken populations and Mayer bond orders for the substrate cluster VnOssHss, see Fig. 12b. The data are obtained from DFT calculations [129] using the RPBE functional [124, 125]. Here positive vanadium ions closest to the surface are described as and the negative oxygen as... 

They called this functional RPBE. So, while the revPBE functional deviates form the PBE functional in the value of one parameter (k) in the exchange enhancement factor Fx(s), the RPBE functional deviates from the PBE functional in the form of the functional itself. It must be pointed out that RPBE preserves all the correct features of the parent PBE model. This functional provides very good chemisorption energies, but has not yet been tested on molecular systems. The behavior of the revPBE and RPBE functionals, with to respect the LO limit is shown in figure 1. 

II gives S = 3/2, in agreement with the resting state of the FeMoco whence Model II was assumed to represent the true model of this cluster. Structures and energies for Model II were calculated self-consistently using the RPBE functional. 

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