nan asiński

Critical evaluation of some computational approaches to the problem of basis set superposition error

The basis set extension (BSE) effects such as primary and secondary basis set superposition errors (BSSE) are discussed on the formal and numerical ground. The symmetry‐adapted perturbation theory of intermolecular forces offers an independent reference point to determine efficacy of some computational approaches aiming at elimination of BSSE. The formal and numerical results support the credibility of the function counterpoise method which dictates that the dimer energy calculated within a supermolecular approach decomposes into monomer energies reproduced with the dimer centered basis set and the interaction energy term which also takes advantage of the full dimer basis. Another consistent approach was found to be Cullen’s ‘‘strictly monomer molecular orbital’’ SCF method [J. M. Cullen, Int. J. Quantum Chem. Symp. 25, 193 (1991)] in which all BSE effects are a priori eliminated. This approach misses, however, the charge transfer component of the interaction energy. The SCF and MP2 results obtained withi...

Many‐body theory of intermolecular induction interactions

The second‐order induction energy in the symmetry‐adapted perturbation theory is expressed in terms of electron densities and polarization propagators at zero frequency of the isolated monomers. This expression is used to derive many‐body perturbation series with respect to the Mo/ller–Plesset type correlation potentials of the monomers. Two expansions are introduced—one based on the standard Mo/ller–Plesset expansion of electron densities and polarization propagators, and the second accounting for the so‐called response or orbital relaxation effects, i.e., for the perturbation induced modification of the monomer’s Fock operators. Explicit orbital formulas for the leading perturbation corrections that correctly account for the response effects are derived through the second order in the correlation potential. Numerical results are presented for several representative van der Waals complexes—a rare gas atom and an ion Ar–Na+, Ar–Cl−, and He–F−; a polar molecule and an ion H2O–Na+ and H2O–Cl−; two polar mol...

Mo/ller–Plesset perturbation theory for van der Waals complexes bound by electron correlation effects: Ground states of the Ar and Mg dimers

We demonstrate that MPPT through fourth order is suitable for studying van der Waals correlation‐bound complexes provided that (a) accurate calculations are carried at the SCF level; (b) efficient basis sets for intersystem correlation effects (i.e., dispersion) are used; and (c) the full counterpoise (CP) method is applied to correct for basis set superposition error (BSSE). Interaction potentials are obtained for Ar2 and Mg2 with extended basis sets that contained up through the f‐symmetry functions. For Ar2 the potential is characterized by R≈7.3 a0 and De∼0.34 mhartree and for Mg2 by Re≈7.4a0 and De∼2.1 mhartree. The discrepancies between our potentials and the most accurate semiempirical and experimental results (Ar2:Re∼7.1a0, De ≈0.45 mhartree; Mg2:Re≈7.35a0, De∼1.93 mhartree, are analyzed in detail and attributed to the lack of higher than f‐symmetry functions, as well as, in the Mg2 case, to the approximate nature of the MP4 approach.

On the role of bond functions in interaction energy calculations: Ar⋅⋅⋅HCl, Ar⋅⋅⋅H2O, (HF)2

We analyze the effect of an extended set of bond functions on the SCF and MP2 interaction energies, and their SAPT perturbation components; electrostatic, induction, dispersion, and exchange. The electrostatic, induction, and exchange terms at the SCF level prove to be largely independent. The dispersion energy is substantially improved and the improvement did not depend much on the bond‐function location. In contrast, the electrostatic‐correlation term is usually seriously distorted and the distortion strongly dependent on the bond‐function location. It was also shown that the distortion may be significantly reduced by appropriate shifting of the location. Only then the interaction energies obtained with bond functions may be considered reliable. It is strongly recommended to control the electrostatic‐correlation term. We also present samples of accurate results (within 5% error bar) for the Ar–HCl, Ar–H2O, and (HF)2 complexes.

Ab initio study of van der Waals interaction of CO2 with Ar

The ab initio potential energy surface of the ArCO2 cluster is calculated using the supermolecular Mo/ller–Plesset perturbation theory (S‐MPPT) and dissected into its fundamental components; electrostatic, exchange, induction, and dispersion energies. The surface contains a single minimum for the perpendicular approach of Ar toward the C atom which has a well depth of ∼210 cm−1 at R=6.5 a0. This value is obtained using an extended basis set supplied with the bond functions and the fourth order supermolecular Mo/ller–Plesset calculations, and is expected to be accurate to within ±5%. The areas of the surface corresponding to the collinear approach of Ar to CO2 contain an extended plateau. The saddle point in this region for R=9.0 a0 is stabilized by 117 cm−1. The analytical pair potential for Ar–CO2 obtained by fitting to the individual interaction components is provided. The three‐body effects in the related cluster, Ar2CO2, are examined for two configurations of the Ar2CO2 cluster. The overall nonadditiv...

Partitioning of interaction energy in van der Waals complexes involving excited state species: The He(1S)+Cl2(B 3Πu) interaction

The partitioning of interaction energy between a closed‐shell and an open‐shell system is proposed. This allows us to describe the unrestricted Mo/ller–Plesset interaction energy as a sum of fundamental contributions: electrostatic, exchange, induction and dispersion. The supermolecular energies derived within unrestricted Mo/ller–Plesset perturbation theory are analyzed in terms of perturbation theory of intermolecular forces. The latter has been generalized to allow for the description of monomer wave functions within the unrestricted Hartree–Fock approach. The method is applied to the potential energy surfaces for the first excited triplet states, 3A′ and 3A″, of the He+Cl2(3Πu) complex. The 3A′ and 3A″ potential energy surfaces have different shapes. The lower one, 3A′, has a single minimum for the T‐shaped structure. The higher one, 3A″, has the global minimum for the T‐shaped structure and the secondary minimum for a linear orientation. The calculated well depth for the 3A′ state is 31.1 cm−1 at the...

Ab initio calculations of the interaction of He with the B 3Π0u+ state of Cl2 as a function of the Cl2 internuclear separation

Ab initio calculations using unrestricted Mo/ller–Plesset perturbation theory to fourth order (UMP-4) were carried out for the interaction of He with the B 3Π0u+ state of Cl2. Also, more reliable unrestricted coupled cluster singles, doubles, and noniterative triples (UCCSD(t)) calculations were performed for several points on the B electronic state surface and were used to scale the UMP-4 points. Exp-6 type two center potential energy functions were fitted to the modified UMP-4 points (B state) to construct an analytical three-dimensional potential energy surface. An r (Cl–Cl separation) dependence was incorporated in the B state potential energy surface to allow the calculation of HeCl2 properties in different vibrational states so that vibrational predissociation rates could be calculated. Excitation spectra, predissociation lifetimes, and rotational product distributions were calculated and compared to the available experimental data. It was found that the calculated B←X, 8←0 spectrum is in good agree...

Ab initio study of the nonadditive effects in the trimer of ammonia

The three‐body potential in the trimer of ammonia is analyzed in terms of Heitler–London (HL)‐exchange, self‐consistent field (SCF)‐deformation, induction, and dispersion nonadditivities. The nonadditive term is due largely to the SCF‐deformation effect. However, its anisotropy resembles more that of the HL‐exchange nonadditivity. Correlation effects do not contribute significantly to the nonadditivity. The trimer is of C3h symmetry. The geometry is determined at the level of pairwise interactions. Apart from a slight shortening of the N––N distance, the three‐body effect has virtually no influence on the mutual orientations of subsystems in the trimer. Nonadditive properties of ammonia are compared to those of other trimers: (HF)3; (HCl)3; (H2O)3; and (CH4)3. An examination of the basis set dependence of the components of three‐body effect leads us to believe that the present treatment yields three‐body potential with accuracy better than 0.1 kcal/mol for the trimer of ammonia, as well as for other polar...

Ab initio study of the O2(X 3Σ−g)+He(1S) van der Waals cluster

Potential energy surface for the He(1S)+O2(X 3Σ−g) interaction is calculated using the supermolecular unrestricted Mo/ller‐Plesset perturbation theory approach and is analyzed via the perturbation theory of intermolecular forces. The latter has been generalized to provide a decomposition of the interaction energies into electrostatic, exchange, induction, and dispersion constituents for monomers described by unrestricted Hartree–Fock determinants. The global minimum occurs for the T‐shaped geometry, around 6.0a0. Our UMP4 estimate of the well depth of the global minimum is De=27.7 cm−1. This value is expected to be accurate to within a few percent. The potential energy surface reveals also a local minimum for the collinear geometry at about 7.0a0. The well depth for the secondary minimum is estimated at De=25.5 cm−1(UMP4) and is expected to be accurate within a few percent. The minima are separated by a barrier of 7.5 cm−1. The energy partitioning reveals that the origin of interaction in this complex is ...

The effect of two- and three-body interactions in ArnCO2 (n=1,2) on the asymmetric stretching CO2 coordinate: An ab initio study

The dependence of the two-body and three-body interactions in the ArnCO2 cluster upon the intramolecular asymmetric stretching coordinate of CO2 is studied by the ab initio method. In the T-shaped binary complex Ar–CO2, the influence of the components of the interaction energy on the shift of the asymmetric stretching frequency of CO2 (ν3) is estimated within a one-dimensional vibrational model and compared with the experimental data of Sperhac, Weida, and Nesbitt [J. Chem. Phys. 104, 2202 (1996)]. The interaction energy is dissected into Heitler–London, induction, and dispersion energies and their respective intrasystem correlation corrections. The redshift represents a delicate balance of these effects on the v=0 and v=1 levels. The highly correlated treatment is required to describe the dependence of two-body potential upon the stretching coordinate. The supermolecular coupled cluster calculations with the single, double, and noniterative triple excitations reproduce the shift observed by Sperhac et al...