On the accuracy of density functional theory for ion–molecule clusters. A case study of PL_n⁺ clusters of the first and second row hydrides

Abstract

PL,,+ clusters (n = 1, 2 and L = NH3, OHz, FH, pH3, SH2, ClH) in both their triplet and singlet states have been characterized by common approximate density functional methods, SVWN, BVWN, BLYP, and B3LYP. The phosphorus-ligand distances (R), dissociation energies (Do), triplet-singlet gaps and several bond properties, such as the electron density (p(rc)), the Laplacian (V2p(rC)), and the local energy density H(rc) at the bond critical point, were compared with those obtained by accurate ab initio molecular orbital theory, namely, second-order MOller-Plesset (MP2) and G2 theory. In general, it is observed that the local spin density approximation (SVWN) yields stronger bonds than ab initio molecular orbital theory. However, addition of gradient corrections to the exchange functional (BVWN) yields ion-molecule bonds that are too weak. Finally, taking account also of gradient corrections to the correlation functional (BLYP) leads to very close agreement with ab initio results. Among these functionals, Becke's hybrid functional, B3LYP, best fit the second-order M~ller-Plesset and G2 data, reproducing the qualitative trends observed for the above-mentioned properties of phosphorus clusters, except for V2p(rc). This fit is particularly good for distances, dissociation energies, and electron densities at the bond critical point, and both methods show similar deviations of the values of binding energies and triplet-singlet gap with respect to the G2 data. Compared with our most accurate ab initio molecular orbital data, namely G2, significant overbinding for the singlets, larger for one-ligand than for two-ligand complexes, and significant overestimation of the triplet-singlet gap for one-ligand complexes is observed for both methods, namely, B3LYP and MP2. The deviations at the second-order MOller-Plesset level of theory are mainly due to the lack of quadratic configuration interaction (QCI) corrections, and this deficiency is also present to some extent in B3LYP. However, for larger clusters these corrections are smaller, therefore the B3LYP functional is expected to lead to accurate descriptions. triplet-singulet pour les complexes un coordinat. Les deviations au niveau de la thCorie de Moiler-Plesset du deuxieme ordre sont principalement dues a un manque de corrections pour I'interaction quadratique de configuration (IQC) et cette dificience se retrouve aussi, jusqu'i un certain point, dans B3LYP. Toutefois, pour les agregats plus volumineux, ces corrections sont plus petites et on peut s'attendre ce que la fonctionnelle B3LYP conduise i des descriptions prkcises.

Density Functional Studies of the bπ.aσ Charge-Transfer Complex Formed between Ethyne and Chlorine Monofluoride

“…Thus, when the modified Becke's term is used, the averaged error in the predicted values of the rotational constants is 3.6%; however, using the standard Becke's exchange gradient correction term, the error raises to 4.9%. The latter figure compares well with the error associated with the MP2 calculated rotational constants, namely, 4.8%, a fact that comes along with the reported close behavior of the BLYP and MP2 methods for both, strongly bound molecules and weakly bound ion−molecule complexes . However, BLYP and MP2 behave oppositely with respect to experiment.…”

Charge density distribution of 3-(1-aminoethylidene)-2-methoxy-2-oxo-2,3-dihydro-2λ5-benzo[e][1,2]oxaphosphinin-4-one

“…However, positive Laplacians are reproducible for the experiment, as mentioned above. On the other hand, many examples exist of theoretical values of Laplacians which are comparable to those we have found here (Lopez et al, 1996;Mõ & Yá ň ez, 1996;Boily, 2002). A thorough investigation of r 2 (BCP) values of oxaphosphinane derivatives optimized previously has revealed comparable values.…”

Density functional studies of the pseudo-π.aσ charge-transfer complex between cyclopropane and chlorine monofluoride

“…This supports the evidence of the quality of the LYP correlation functional in describing weakly bound complexes. 29 Inclusion of part of the HartreeÈFock exchange, through the hybrid methods B3X (X \ P86, PW91, LYP), greatly improves the agreement between experiment and theory, as shown by inspection of the bottom half of Table 1. In particular, the agreement with respect to the experimental values of the calculated rotational constants is remarkable.…”