Plesset Perturbation Theory Level Research Articles

Efficient low-scaling computation of NMR shieldings at the second-order Møller-Plesset perturbation theory level with Cholesky-decomposed densities and an attenuated Coulomb metric.

A method for the computation of nuclear magnetic resonance (NMR) shieldings with second-order Møller-Plesset perturbation theory (MP2) is presented which allows to efficiently compute the entire set of shieldings for a given molecular structure. The equations are derived using Laplace-transformed atomic orbital second-order Møller-Plesset perturbation theory as a starting point. The Z-vector approach is employed for minimizing the number of coupled-perturbed self-consistent-field equations that need to be solved. In addition, the method uses the resolution-of-the-identity approximation with an attenuated Coulomb metric and Cholesky decomposition of pseudo-density matrices. The sparsity in the three-center integrals is exploited with sparse linear algebra approaches, leading to reduced computational cost and memory demands. Test calculations show that the deviations from NMR shifts obtained with canonical MP2 are small if appropriate thresholds are used. The performance of the method is illustrated in calculations on DNA strands and on glycine chains with up to 283 atoms and 2864 basis functions.

Relations between the aromaticity and magnetic dipole transitions in the electronic spectra of hetero[8]circulenes.

Magnetically induced current densities have been calculated at the second-order Møller-Plesset perturbation theory (MP2) level for seven hetero[8]circulenes and their dicationic and dianionic forms. Calculations of the magnetic dipole transition moments have also been carried out at the algebraic diagrammatic construction (ADC(2)) and the second-order approximate coupled-cluster (CC2) levels. The calculations show that the degree of aromaticity and the size of the magnetic dipole transition moment of the lowest magnetic-dipole allowed excited state are related. We show that neutral hetero[8]circulenes are weakly antiaromatic when the first excited state with a large magnetic dipole transition moment of 10-16 a.u. lies at high energies (∼2.8-3.5 eV). For the dications, this transition often lies at much lower energies. Hetero[8]circulene dications with large magnetic dipole transition moments are strongly antiaromatic. The lowest excited states of the hetero[8]circulene dianions have very small magnetic dipole transition moments implying that they are aromatic.

Theoretical and experimental investigation for SO3 production in SO2-rich astrophysical environments

SynopsisThis work presents the results for the irradiation of pure SO2 sample that was condensed in a preevacuated chamber, from Laboratório de Astroquímica e Astrobiologia (LASA/UNIVAP), at low temperature (12 K) and irradiated by ionizing photons which simulate Solar photons in the vacuum ultraviolet (VUV) and soft X-rays range. The infrared spectra of irradiated sample have presented the formation of SO3. Experimental formation cross section was determined. Theoretical investigations were performed at Second-order Moller- Plesset perturbation theory (MP2) level and indicate the most likely SO3 formation channels vary with the reaction supporting medium.

Computational studies of a paramagnetic planar dibenzotetraaza[14]annulene Ni(II) complex.

A square-planar Ni(II) dibenzotetraaza[14]annulene complex substituted with two 3,3-dimethylindolenine groups in the meso positions has recently been synthesized and characterized experimentally. In the solid-state, the Ni(II) complex forms linear π-interacting stacks with Ni···Ni separations of 3.448(2) Å. Measurements of the temperature dependence of the magnetic susceptibility revealed a drastic change in the magnetic properties at a temperature of 13 K, indicating a transition from low-to-high spin states. The molecular structures of the free-base ligand, the lowest singlet, and triplet states of the monomer and the dimer of the Ni complex have been studied computationally using density functional theory (DFT) and ab initio correlation levels of theory. In calculations at the second-order Møller-Plesset (MP2) perturbation theory level, a large energy of 260 kcal mol(-1) was obtained for the singlet-triplet splitting, suggesting that an alternative explanation of the observed magnetic properties is needed. The large energy splitting between the singlet and triplet states suggests that the observed change in the magnetism at very low temperatures is due to spin-orbit coupling effects originating from weak interactions between the fine-structure states of the Ni cations in the complex. The lowest electronic excitation energies of the dibenzotetraaza[14]annulene Ni(II) complex calculated at the time-dependent density functional theory (TDDFT) levels are in good agreement with values deduced from the experimental UV-vis spectrum. Calculations at the second-order algebraic-diagrammatic construction (ADC(2)) level on the dimer of the meso-substituted 3,3-dimethylindolenine dibenzotetraaza[14] annulene Ni(II) complex yielded Stokes shifts of 85-100 nm for the lowest excited singlet states. Calculations of the strength of the magnetically induced ring current for the free-base 3,3-dimethylindolenine-substituted dibenzotetraaza[14]annulene show that the annulene ring is very weakly antiaromatic, sustaining a paratropic ring-current strength of only -1.7 nA/T.

Towards the versatile DFT and MP2 computational schemes for 31P NMR chemical shifts taking into account relativistic corrections.

The main factors affecting the accuracy and computational cost of the calculation of (31)P NMR chemical shifts in the representative series of organophosphorous compounds are examined at the density functional theory (DFT) and second-order Møller-Plesset perturbation theory (MP2) levels. At the DFT level, the best functionals for the calculation of (31)P NMR chemical shifts are those of Keal and Tozer, KT2 and KT3. Both at the DFT and MP2 levels, the most reliable basis sets are those of Jensen, pcS-2 or larger, and those of Pople, 6-311G(d,p) or larger. The reliable basis sets of Dunning's family are those of at least penta-zeta quality that precludes their practical consideration. An encouraging finding is that basically, the locally dense basis set approach resulting in a dramatic decrease in computational cost is justified in the calculation of (31)P NMR chemical shifts within the 1-2-ppm error. Relativistic corrections to (31)P NMR absolute shielding constants are of major importance reaching about 20-30 ppm (ca 7%) improving (not worsening!) the agreement of calculation with experiment. Further better agreement with the experiment by 1-2 ppm can be obtained by taking into account solvent effects within the integral equation formalism polarizable continuum model solvation scheme. We recommend the GIAO-DFT-KT2/pcS-3//pcS-2 scheme with relativistic corrections and solvent effects taken into account as the most versatile computational scheme for the calculation of (31)P NMR chemical shifts characterized by a mean absolute error of ca 9 ppm in the range of 550 ppm.

What is the most efficient way to reach the canonical MP2 basis set limit?

Various ways of reaching the complete basis set limit at the second-order Møller–Plesset perturbation theory (MP2) level are compared with respect to their cost-to-accuracy ratio. These include: (1) traditional MP2 calculations with correlation consistent basis sets of increasing size, with and without the resolution of identity for Coulomb and exchange (RIJK) or the combined RIJ and ‘chain of spheres’ (RIJCOSX) approximations; (2) basis set extrapolation obtained with the same MP2 variants; and (3) explicitly correlated F12-MP2 methods. The time required to solve the Hartree–Fock equations is part of the evaluation because the overall efficiency is of central interest in this work. Results were obtained for the ISO34, DC9 and S66 test sets and were analysed in terms of efficiency and accuracy for total energies, reaction energies and their effect on the basis set superposition error. Among the methods studied, the RIJK-MP2-F12 and RIJK-MP2-EP1 (where EP1 stands for ‘Extrapolation Protocol 1’ as explained in the text) methods perform outstandingly well. Although extrapolation is, in general, slightly faster than explicit correlation, it is found that for reaction energies, RIJK-MP2-F12 performs systematically better. This holds especially in combination with a triple zeta basis set, in which case it even outperforms the much more costly extrapolation involving quadruple- and quintuple-zeta correlation consistent basis sets.

Analysis of Infrared Spectra of Neat Liquid N-Methylpyrrole

The vibrational properties of neat liquid N-methylpyrrole were investigated. Quantitative near-infrared and mid-infrared spectra recordings including thin lm MIR transmission studies were performed. Basing on these data the optical constants of N-methylpyrrole in the 12500 560 cm−1 region were determined. The experiment was supported by an anharmonic analysis based on Moller Plesset perturbation theory and density functional theory level of theory, including the hybrid B2PLYP functional. Assignments of numerous bands observed in the liquid phase were proposed basing on potential energy distribution calculations and a comparison of experimental and simulated spectra was presented. The inaccuracy of static calculations in the reproduction of experimental spectrum in the C H stretching region has been observed and explained by the rotation of methyl group. A vibrational spectrum was therefore calculated by the use of Car Parinello molecular dynamics method for a better simulation of experimental spectrum. DOI: 10.12693/APhysPolA.124.115 PACS: 78.20.Bh, 78.20.Ci, 78.30.cb

Structures, molecular orbitals and UV–vis spectra investigations on Br2C6H4: A computational study

The dibromobenzenes (1,2-, 1,3- and 1,4-Br2C6H4) have been studied by theoretical methods. The structures of these species are optimized and the structural characteristics are determined by density functional theory (DFT) and the second order Møller–Plesset perturbation theory (MP2) levels. The geometrical structures of Br2C6H4 show a little distortion of benzene ring due to the substitution of highly electronegativity of bromine atoms. The electronegativity of bromine atoms in 1,4-Br2C6H4 is predicted to be more negative than 1,2- and 1,3-Br2C6H4. In addition, dipole moment and frontier molecular orbitals (FMOs) of these Br2C6H4 are performed as well. The 1,4-Br2C6H4 is slightly more reactive than 1,2- and 1,3-Br2C6H4 because of its small HOMO–LUMO energy gap. The simulated UV–vis spectra are investigated by time-dependent density functional theory (TD-DFT) approach, which are in excellent agreement with the available experimental value. Our calculations show that a few of absorption features are between 140nm and 250nm, which is in ultraviolet C range, and the red shift of 1,3- and 1,4-Br2C6H4 are predicted. Moreover, the UV absorption features of these Br2C6H4 in water or methanol are predicted to be more intense than in gas phase due to solvent effect.

A novel hexanuclear silver(i) cluster containing a regular Ag6 ring with short Ag–Ag distances and an argentophilic interaction

The hexanuclear complex [HQ][Ag(p-mpspa)] (H2-p-mpspa = 3-(4-methoxyphenyl)-2-sulfanylpropenoic acid) was prepared by reacting the precursor [Ag(H-p-mpspa)] with diisopropylamine (Q). The complex was characterized by spectroscopic techniques and the structure was solved by a single crystal X-ray diffraction study. The crystal contains hydrogen-bonded diisopropylammonium cations and [Ag6(p-mpspa)6](6-) anions that are based on a regular Ag6 ring with each S-donor atom of the sulfanylcarboxylate ligand bridging two Ag atoms. The Ag-Ag bond distances, 2.8036(6) Å, are very short and suggest a closed shell d(10)···d(10) argentophilic interaction. To analyze the relative role of this interaction and that of the S-bridging atom the anionic [Ag6(p-mpspa)6](6-) moiety has been studied theoretically at the Hartree-Fock (HF) and 2(nd) order Møller-Plesset perturbation theory (MP2) levels on a very simple [Ag6(SH)6] A model system. A large model system [Ag6(p-mpspa)6](6-)B has also been studied by applying the ONIOM (QM/MM) approach using HF/UFF and MP2/UFF combinations as levels of theory. The six experimentally observed Ag(I)···Ag(I) supported interactions are reproduced when dispersion-type interactions are considered in the theory levels MP2 and ONIOM MP2/UFF for models A and B, respectively. The use of HF and ONIOM HF/UFF levels led to a similar hexanuclear structure but displayed a large hexagonal disposition without argentophilic contacts for both models A and B. The steric hindrance exerted by the ligands did not preclude the formation of argentophilic interactions, as observed experimentally.

Crystal Polymorphism in Oxalyl Dihydrazide: Is Empirical DFT-D Accurate Enough?

Crystalline oxalyl dihydrazide has five experimentally known polymorphs whose energetics are governed by subtle balances between intra- and intermolecular interactions, providing a severe challenge for theoretical crystal structure modeling. Previous work has shown that many common density functional methods that neglect van der Waals dispersion cannot correctly describe this system, but it has been argued that empirically dispersion-corrected DFT-D performs much better. Here, we examine these crystals with second-order Møller-Plesset perturbation theory (MP2) and related levels of theory using the fragment-based hybrid many-body interaction method. The energetics prove sensitive to the treatment of electron-electron correlation, the basis set, many-body induction, three-body dispersion, and zero-point contributions. Nevertheless, our best predictions for the polymorph energy ordering based on dispersion-corrected MP2C calculations agree with the available experimental data. In contrast, lower levels of theory, including the common B3LYP-D* and D-PW91 dispersion-corrected density functional approximations, fail to reproduce experimental observations and/or the high-level calculations.

Hydrogen bonds between pyrazine and RCOOH (R = H, CH3, and C6H5): A theoretical study

AbstractTheoretical calculations using Møller–Plesset perturbation theory (MP2) levels of calculation with a 6‐31++G(d,p) basis set have been performed to determine stable structures and molecular properties for the 1:1 and 1:2 hydrogen complexes involving pyrazine as proton acceptor and RCOOH (R = H, CH3, and C6H5). Six complexes have been characterized as minima as no imaginary frequency was found. As expected, the HO chemical bonds lengths are increased after complexation. For the strongly bound complex, the doubly complexed pyrazine acts as a bridge between the two end molecules while transferring electrons. The vibrational changes that take place in the RCOOH after complexation with pyrazine follow the usual behavior, and the HO stretching frequency is shifted downward whereas its irradiated intensity is much enhanced. As unexpected result, the vibrational modes of the CH stretching of pyrazine has its frequencies shifted to the blue. © 2012 Wiley Periodicals, Inc.

Methyl and Pentyl Chloride in a Microhydrated Environment and at the Liquid Water−Vapor Interface: A Theoretical Study

The solvation properties of methyl and pentyl chloride were studied in a microhydrated environment with up to 10 explicit water molecules and at the liquid water-vapor interface. Geometry optimizations were performed in the former case using the density functional based tight binding (DFTB), DFTB-D, and Møller-Plesset perturbation theory (MP2) levels of theory. The microhydrated alkyl chloride complexes were characterized in terms of hydrogen bonding and energetic stability. The DFTB and DFTB-D results were verified by comparison with those obtained by MP2. Good agreement between the MP2 and DFTB-D results is found. Complexes where the alkyl chloride molecule is attached to an edge of the water cluster are found to be most stable. Pronounced stability is also observed for cubic arrangements of the alkyl chloride-water complexes. Molecular dynamics simulations based on the DFTB and DFTB-D methods were used to study the adsorption process of the alkyl chloride molecules to a water surface. The dynamics simulations show that the methyl chloride molecule is located at the water surface preferentially with the methyl group oriented toward the water surface, while for pentyl chloride, owing to the longer nonpolar hydrocarbon chain, a parallel alignment at the water surface is found with the hydrocarbon chain pointing slightly to the gas phase. Despite some quantitative differences, the present work provides confirmation of the somewhat surprising preferential orientation of the methyl chloride molecule at the water-vapor interface predicted in a recent study using molecular dynamics simulations based on an empirical force field (Harper et al., J. Phys. Chem. A 2009, 113, 2015-2024). The observed difference in preferred alignment at the aqueous surface between the methyl chloride and the longer-chain alkyl chloride is likely to have consequences for the chemistry of alkyl halides adsorbed on the surface of aqueous and ice particles in the atmosphere.

Quantum chemical predictions of IR spectra and thermodynamic properties of tetrazole derivates

We have predicted the optimized geometries, infrared spectrum, and thermodynamic properties for six tetrazole derivates at density functional theory and second-order Moller–Plesset perturbation theory (MP2) level with the 6-31G** basis set. Their heats of formation were calculated accurately using the designed isodesmic and isogyric reactions. The computed total energies, heat of formation, and enthalpy of combustion consistently indicate the stability: 5-(2,4,6-trinitrophenyl)-2H-tetrazole > 5-(2,4,6-trinitrophenyl)-1H-tetrazole > 5-(2,4,6-trinitrophenyl)-5H-tetrazole The similar result for the isomers of 5-(2,4,6-trinitrobenzyl)-tetrazole: 5-(2,4,6-trinitrophenyl)-2H-tetrazole > 5-(2,4,6-trinitrophenyl)-1H-tetrazole > 5-(2,4,6-trinitrophenyl)-5H-tetrazole.

Computational study of the electronic structure characterization of a novelanti-inflammatory tripeptide derived from monocyte locomotion inhibitoryfactor (MLIF)-pentapeptide

The structure and properties of molecules are determined by their charge-density distribution. Several works have shown that electron delocalization along the peptide backbone and side-chain modulates the physical and chemical features of peptides and protein properties. Research on Entamoeba histolytica-soluble factors led to the identification of the pentapeptide Met-Gln-Cys-Asn-Ser, with anti-inflammatory in vivo and in vitro effects. A synthetic pentapeptide, Met-Pro-Cys-Asn-Ser, maintained the same anti-inflammatory actions in experimental assays. A previous theoretical study allowed proposing the Cys-Asn-Ser tripeptide (CNS tripeptide) as the pharmacophore group of both molecules. This theoretical hypothesis was recently confirmed experimentally. The aim of this study was to characterize the electronic structure and physico-chemical properties of the CNS tripeptide through a theoretical study at the second-order Møller–Plesset perturbation theory (MP2) and density functional theory (DFT) theoretical levels. Our results in deprotonation energies show that the hydrogen atom (H2) of the serine-amide group possesses acidic characteristics. This result was confirmed by means of a study of bond order. Atomic charges, dipole moment, frontier molecular orbitals (Highest occupied molecular orbital [HOMO − 1] and Lowest unoccupied molecular orbital [LUMO + 1]), and electrostatic potential isosurface and its geometric parameters permitted to characterize its electronic structure and physico-chemical features and to identify some reactive sites that could be associated with this tripeptide's anti-inflammatory activity.

Cis–trans isomerization and spin multiplicity dependences on the static first hyperpolarizability for the two-alkali-metal-doped saddle[4]pyrrole compounds

Doping two alkali-metal atoms (Li and Na) into the saddle-shaped saddle[4]pyrrole forms four new two-alkali-metal-doped compounds with alkalide or electride characteristic. They are cis-LiNa(saddle[4]pyrrole) isomers 1(singlet) and 2(triplet), and trans-Li(saddle[4]pyrrole)Na isomers 3(singlet) and 4(triplet). The four structures with all-real frequencies are obtained at the density functional theory (DFT) B3LYP/6-311+G(d) level. All calculations of electric properties have been carried out at the second order Moller–Plesset perturbation theory (MP2) level. The order of the β 0 values is 3.54 × 103 for trans-4(triplet) <1.51 × 104 for cis-1(singlet) <3.57 × 104 for cis-2(triplet) <2.34 × 105 a.u. for trans-3(singlet). The static first hyperpolarizability (β 0) depends on the cis–trans isomerization and spin multiplicity. The result demonstrates that the cis–trans isomerization and spin multiplicity controls of the second-order NLO response are possible.

Quantum-Chemical Comprehensive Study of the Organophosphorus Compounds Adsorption on Zinc Oxide Surfaces

Ab initio calculations at the density functional theory and the second-order Moller−Plesset perturbation theory levels, and using the n-layered integrated molecular orbital and molecular mechanics (ONIOM) method, have been performed for the adsorption of dimethyl methylphosphonate (DMMP) and tabun (GA) on (ZnO)n (where n = 4, 18, or 24 molecular clusters of (1010) and (0001) zinc oxide surfaces). Different adsorption sites and DMMP orientations were considered. Calculations include the evaluation of the optimized geometries, atomic charges, interaction energies, various methods applied, and different sizes and surface types of the ZnO fragments. On both surfaces, the molecular adsorption proceeds as chemisorption via the formation of a Zn···O chemical bond in the case of the DMMP adsorption complex and a P···O covalent bond or a Zn···N chemical bond for GA adsorption complexes. The type of surface greatly affected the strength of the intermolecular interactions and the interaction energies. The results in...

Large static first and second hyperpolarizabilities dominated by excess electron transition for radical ion pair salts M2˙+TCNQ˙−(M = Li, Na, K)

The interesting radical ion pair salts M(2)*(+)TCNQ*(-) (M=Li, Na, K) are a particular class of charge transfer complexes with excess electron. The ground states of these complexes are triplet. The C(2v) symmetry geometrical structures of the M(2)*(+)TCNQ*(-) (M=Li, Na, K) with all-real frequencies are obtained at the density functional theory (DFT) B3LYP/6-31+G(d) level. All calculations of electric properties in this paper have been carried out at the restricted open-shell second order Møller-Plesset perturbation theory (ROMP2) level. Owing to existing excess electron (from the polarized alkali metal atoms) these charge transfer complexes exhibit large nonlinear optical (NLO) responses dominated by excess electron transitions.For these radical ion pair salts M(2)*(+)TCNQ*(-), the static first hyperpolarizabilities (beta(0)) are large. The order of beta(0) values is 19 203 (M=Li)<24 140 (M=Na) < 29 065 a.u. (M=K). Specially, the second hyperpolarizability (gamma(0)) of the complexes with excess electron is obtained for the first time. These static second hyperpolarizabilities are also large. The order of gamma(0) values is 2,213,006 (M=Li)<3,136,754 (M=Na)<7,905,623 a.u. (M = K). Among the three structures, K(2)*(+)TCNQ*(-) has the largest gamma(0) value to be 7.9 x 10(6) a.u. (3982 x 10(-36) esu), which is about 9 times larger than that of the intramolecular charge transfer complex sigma-arylvinylidene trans-[Ru(4-C[double bond, length as m-dash]CHC(6)H(4)C[triple bond, length as m-dash]CC(6)H(4)NO(2))Cl(dppm)(2)]PF(6) [Hurst et al., Organometallics, 2001, 20, 4664]. The present investigation provides a new kind of candidates for the high-performance NLO materials.

Electric properties of the azide ion calculated with a systematic sequence of Gaussian-type basis sets

We have designed a sequence of flexible Gaussian-type basis sets for electric property calculations on the azide ion. These basis sets range from [7s5p2d] to [7s5p5d3f2g] size. We have analyzed the performance of these basis sets on calculations of the quadrupole ( Θ), hexadecapole ( Φ) moment, the mean ( α ¯ ) and the anisotropy (Δ α) of the dipole polarizability at the self-consistent field (SCF), second-order Møller–Plesset perturbation theory (MP2) and B3LYP levels of theory. Our evaluation of the quality of these purpose-oriented basis sets bears on their overall performance on the { Θ , Φ , α ¯ , Δ α } collection of properties. Our best values for each property at MP2/[7s5p5d3f2g] are: Θ = - 8.5953 e a 0 2 , Φ = - 7.50 e a 0 4 , α ¯ = 41.74 and Δ α = 38.24 e 2 a 0 2 E h - 1 .

Pseudospin Hamiltonian parameters evaluated with regard to alkali metal ions in K3(H/D)(SO4)2 materials

AbstractAn analysis of ferroelectric behavior of K3(H/D)(SO4)2 as members of the M3(H/D)(AO4)2 family is examined quantum chemically in the cluster approach using the structural models extended through the incorporation of K ions in an explicit form. We apply several structural models that describe the different types of (H/D)(SO4)2‐dimer surrounded by K ions, the geometry of the dimer being borrowed from the corrected diffraction data of the K3D(SO4)2 crystal in its ordered phase at the temperature below critical point Tc = 85 K. The calculations of the pseudospin Hamiltonian parameters—the Ising coupling parameters Jij and tunneling integrals Ω—were performed on the Hartree–Fock and second‐order Møller–Plesset perturbation theory levels with the 6‐31G–type basis sets. It is demonstrated that an effect of the K ions incorporation on the Jij values is minor (about several Kelvins). At the same time an influence of these ions on the double‐well potential characteristics (particularly, on the barrier heights) and thus on the tunneling integral value is more pronounced for both D‐ and H‐containing species. Despite the changes of the Hamiltonian parameters found, the major conclusion obtained earlier, referring to the suppression of the structural phase transition in H‐containing specimens due to proton quantum fluctuations, remains valid. The numerical estimations of the critical temperature Tc for completely deuterated materials that are obtained with and without regard to alkali metal ions are discussed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Theoretical study of hydrogen bonds between acetylene and selected proton donor systems

AbstractThe equilibrium structures, the binding energies, and the second‐order energy components of a series of hydrogen‐bonded complexes involving acetylene are studied. The strength of the binding energy of the selected systems (HF … HCCH, HCl … HCCH, HCN … HCCH, and HCCH … HCCH) was different, ranging from a very weak interaction to a strong interaction. Calculations have been carried out at both the Hartree–Fock and correlated (second‐order Møller–Plesset perturbation theory) levels of theory, using several different basis sets [6‐31G(d,p), 6‐311G(d,p), 6‐31G++(d,p), 6‐311G++(d,p), 6‐31++G(2d,2p) and 6‐311++G(2d,2p)]. The widely used a posteriori Boys–Bernardi counterpoise (CP) correction scheme has been compared with the a priori CHA/CE, CHA–MP2, and CHA–PT2 methods, using the chemical Hamiltonian approach (CHA). The results show that at both levels the CP and the appropriate CHA results are very close to each other. Only the monomer‐based CHA‐PT2 theory gives slightly overcorrected results, reflecting that the charge transfer and polarization effects are not taken into account in this method up to second order. We have also applied our earlier developed energy decomposition scheme in order to decompose the second‐order energy contribution into different physically meaningful components. The results show that at large and intermediate intermolecular distances, the second‐order intermolecular contribution is almost equal to the sum of different physically meaningful components (e.g., polarization, charge transfer, dispersion), while at shorter distances the slightly strong overlap effects fairly disturb this simple additivity. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005