Masuhiro Mikami

Effects of the higher electron correlation correction on the calculated intermolecular interaction energies of benzene and naphthalene dimers: Comparison between MP2 and CCSD(T) calculations

Abstract Intermolecular interaction energies of parallel and T-shape benzene dimers and parallel naphthalene dimer were calculated with MP2, MP3, MP4(SDQ), MP4(SDTQ), CCSD and CCSD(T) electron correlation corrections using several basis sets. The MP2 calculations considerably overestimated the attraction compared to the CCSD(T) ones. The MP2 correlation interaction energies, the differences between the HF and MP2 interaction energies, were 21–38% larger than the corresponding CCSD(T) ones. The MP4(SDQ) and CCSD calculations substantially underestimated the attraction compared to MP4(SDTQ) and CCSD(T), which indicated the importance of triple excitation. The estimated CCSD(T) interaction energies of the three dimers with reasonably large basis sets were −1.74, −2.50 and −5.69 kcal/mol, respectively.

High-level ab initio computations of structures and interaction energies of naphthalene dimers: Origin of attraction and its directionality

The intermolecular interaction energies of naphthalene dimers have been calculated by using an aromatic intermolecular interaction model (a model chemistry for the evaluation of intermolecular interactions between aromatic molecules). The CCSD(T) (coupled cluster calculations with single and double substitutions with noniterative triple excitations) interaction energy at the basis set limit has been estimated from the second-order Møller-Plesset perturbation interaction energy near saturation and the CCSD(T) correction term obtained using a medium-size basis set. The estimated interaction energies of the set of geometries explored in this work show that two structures emerge as being the lowest energy, and may effectively be considered as isoenergetic on the basis of the errors inherent in out extrapolation procedure. These structures are the slipped-parallel (Ci) structure (-5.73 kcal/mol) and the cross (D2d) structure (-5.28 kcal/mol). The T-shaped (C2v) and sandwich (D2h) dimers are substantially less stable (-4.34 and -3.78 kcal/mol, respectively). The dispersion interaction is found to be the major source of attraction in the naphthalene dimer. The electrostatic interaction is substantially smaller than the dispersion interaction. The large dispersion interaction is the cause of the large binding energies of the cross and slipped-parallel dimers.

Molecular Dynamics Simulations of Ionic Liquids: Cation and Anion Dependence of Self-Diffusion Coefficients of Ions

Molecular dynamics simulations of a series of ionic liquids [1-alkyl-3-methylimidazolium (alkyl = methyl, ethyl, butyl, hexyl, and octyl), 1-butylpyridinium, N-butyl-N,N,N-trimethylammonium and N-butyl-N-methylpyrrolidinium cations combined with a (CF(3)SO(2))(2)N(-) anion ([mmim][TFSA], [emim][TFSA], [bmim][TFSA], [C(6)mim][TFSA], [C(8)mim][TFSA], [bpy][TFSA], [(n-C(4)H(9))(CH(3))(3)N][TFSA], and [bmpro][TFSA]) and a 1-butyl-3-methylimidazolium combined with BF(4)(-), PF(6)(-), CF(3)CO(2)(-), CF(3)SO(3)(-), and (C(2)F(5)SO(2))(2)N(-) anions ([bmim][BF(4)], [bmim][PF(6)], [bmim][CF(3)CO(2)], [bmim][CF(3)SO(3)], and [bmim][BETA])] were carried out using the OPLS force field for ionic liquids. The force field was refined on the basis of ab initio molecular orbital calculations of isolated ions and experimental densities for four ionic liquids. The densities calculated for the 13 ionic liquids agreed with the experimental values within a 2% error. The self-diffusion coefficients calculated for the ions in the 13 ionic liquids were compared with the experimental values obtained by the NMR measurements. Although the calculated self-diffusion coefficients were about 1 order smaller than the experimental ones, the cation and anion dependence (the effects of alkyl chain length in imidazolium, cation structures, and anion species) of the experimental self-diffusion coefficients was reproduced by the simulations quite well in most cases. The translational motion of the terminal carbon atoms in the alkyl chains of the imidazolium cations on the time scale of a few nanoseconds is significantly faster than that of the atoms in the imidazolium rings and anions, which suggests that the dynamics of atoms in the polar domains of the ionic liquids is significantly different from that in the nonpolar domains. The factors determining the self-diffusion coefficients of the ions are also discussed.

Theoretical analysis of the hydrogen bond of imidazolium C2–H with anions

The intermolecular interaction energies of ion pairs of imidazolium-based ionic liquids were studied by MP2/6-311G level ab initio calculations. Although the hydrogen bond between the C(2) hydrogen atom of an imidazolium cation and anion has been regarded as an important interaction in controlling the structures and physical properties of ionic liquids as in the cases of conventional hydrogen bonds, the calculations show that the nature of the C(2)-H...X interaction is considerably different from that of conventional hydrogen bonds. The interaction energies of the imidazolium cation with neighboring anions in the four crystals of ionic liquids were calculated. The size of the interaction is determined mainly by the distance between the imidazolium ring and anion. The calculated interaction energy is nearly inversely proportional to the distance, which shows that the charge-charge interaction is the dominant interaction in the attraction. The orientation of the anion relative to the C(2)-H bond does not greatly affect the size of the interaction energy. Calculated interaction energy potentials of 1,3-dimethylimidazolium tetrafluoroborate ([dmim][BF(4)]) complexes show that the C(2)-H bond does not prefer to point toward a fluorine atom of the BF(4). This shows that the C(2)-H...X hydrogen bond is not essential for the attraction.

Replica-exchange Monte Carlo method for the isobaric–isothermal ensemble

Abstract We propose an extension of replica-exchange Monte Carlo method for canonical ensemble to isothermal–isobaric ensemble as an effective method to search for stable states quickly and widely in complex configuration space. We investigated the efficiency of the new method on a benchmark testing system which consists of 256 Lennard-Jones particles. The new method enables us to shorten dramatically the relaxation time of phase change from liquid structure to crystal structure in comparison with the conventional Monte Carlo method.

Effects of basis set and electron correlation on the calculated interaction energies of hydrogen bonding complexes: MP2/cc-pV5Z calculations of H2O–MeOH, H2O–Me2O, H2O–H2CO, MeOH–MeOH, and HCOOH–HCOOH complexes

The MP2 intermolecular interaction energies of the title complexes were calculated with the Dunning’s correlation consistent basis sets (cc-pVXZ, X=D, T, Q, and 5) and the interaction energies at the basis set limit were estimated. The second-order Mo/ller–Plesset (MP2) interaction energies greatly depend on the basis sets used, while the Hartree–Fock (HF) energies do not. Small basis sets considerably underestimate the attractive interaction. The coupled cluster single double triple [CCSD(T)] interaction energies are close to the MP2 ones. The expected CCSD(T) interaction energies of the H2O–MeOH, H2O–Me2O, H2O–H2CO, MeOH–MeOH, and HCOOH–HCOOH complexes at the basis set limit are −4.90, −5.51, −5.17, −5.45, and −13.93 kcal/mol, respectively, while the HF/cc-pV5Z energies are −3.15, −2.58, −3.60, −2.69, and −11.29 kcal/mol, respectively. The HF calculations greatly underestimate the attractive energies and fail to predict the order of the bonding energies in these complexes. These results show that a larg...

Torsional potential of biphenyl: Ab initio calculations with the Dunning correlation consisted basis sets

The internal rotational barrier heights of biphenyl were calculated with the Dunning correlation consisted basis sets (up to cc-pVQZ, 960 basis functions) and the electron correlation correction by the second order Mo/ller-Plesset method (MP2). Although previous Hartree–Fock (HF) and MP2 calculations showed that the internal rotational barrier height at 0° (ΔE0) was substantially larger than that at 90° (ΔE90), our MP2/cc-pVQZ//MP2/6-31G* calculations showed that ΔE0 (2.28 kcal/mol) was close to ΔE90 (2.13 kcal/mol), which agreed with the estimation from experimental measurements. The calculations of benzene dimers suggested that the dispersion interaction increased the relative stability of the coplanar conformer. The basis sets employed in the previous calculations were not large enough to evaluate the attractive dispersion interaction. The underestimation of the stabilization of the coplanar conformer by the dispersion interaction would be one of the reasons for the overestimation of ΔE0 in the previou...

Estimated MP2 and CCSD(T) interaction energies of n-alkane dimers at the basis set limit: Comparison of the methods of Helgaker et al. and Feller

The MP2 (the second-order Møller-Plesset calculation) and CCSD(T) (coupled cluster calculation with single and double substitutions with noniterative triple excitations) interaction energies of all-trans n-alkane dimers were calculated using Dunnings [J. Chem. Phys. 90, 1007 (1989)] correlation consistent basis sets. The estimated MP2 interaction energies of methane, ethane, and propane dimers at the basis set limit [EMP2(limit)] by the method of Helgaker et al. [J. Chem. Phys. 106, 9639 (1997)] from the MP2/aug-cc-pVXZ (X=D and T) level interaction energies are very close to those estimated from the MP2/aug-cc-pVXZ (X=T and Q) level interaction energies. The estimated EMP2(limit) values of n-butane to n-heptane dimers from the MP2/cc-pVXZ (X=D and T) level interaction energies are very close to those from the MP2/aug-cc-pVXZ (X=D and T) ones. The EMP2(limit) values estimated by Fellers [J. Chem. Phys. 96, 6104 (1992)] method from the MP2/cc-pVXZ (X=D, T, and Q) level interaction energies are close to those estimated by the method of Helgaker et al. from the MP2/cc-pVXZ (X=T and Q) ones. The estimated EMP2(limit) values by the method of Helgaker et al. using the aug-cc-pVXZ (X=D and T) are close to these values. The estimated EMP2(limit) of the methane, ethane, propane, n-butane, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane dimers by the method of Helgaker et al. are -0.48, -1.35, -2.08, -2.97, -3.92, -4.91, -5.96, -6.68, -7.75, and -8.75 kcal/mol, respectively. Effects of electron correlation beyond MP2 are not large. The estimated CCSD(T) interaction energies of the methane, ethane, propane, and n-butane dimers at the basis set limit by the method of Helgaker et al. (-0.41, -1.22, -1.87, and -2.74 kcal/mol, respectively) from the CCSD(T)/cc-pVXZ (X=D and T) level interaction energies are close to the EMP2(limit) obtained using the same basis sets. The estimated EMP2(limit) values of the ten dimers were fitted to the form m0+m1X (X is 1 for methane, 2 for ethane, etc.). The obtained m0 and m1 (0.595 and -0.926 kcal/mol) show that the interactions between long n-alkane chains are significant. Analysis of basis set effects shows that cc-pVXZ (X=T, Q, or 5), aug-cc-pVXZ (X=D, T, Q, or 5) basis set, or 6-311G** basis set augmented with diffuse polarization function is necessary for quantitative evaluation of the interaction energies between n-alkane chains.

Rapid calculation of two-dimensional Ewald summation

A computationally efficient method was developed to implement the Ewald summation in calculations for Coulomb interactions in three-dimensional (3D) systems with two-dimensional (2D) periodicity. The computational efficiency and accuracy of this method was evaluated for water systems enclosed in various rectangular parallelepiped boxes (cube, quadratic prism, and slab) and with 2D periodicity. Compared with existing computational methods, this method showed a significant reduction in computational time for all systems examined and was sufficiently accurate for calculating the Coulomb interactions in 3D systems with 2D periodicity, independent of the shape of the simulation box.

Energy profile of the interconversion path between T-shape and slipped-parallel benzene dimers

The energy profile of the interconversion path between the T-shape and slipped-parallel dimers has been studied by high level ab initio calculations. The CCSD(T) (coupled cluster calculation with single and double substitutions with noniterative triple excitations) interaction energy at the basis set limit has been estimated from the MP2 (the second-order Moller–Plesset calculation) interaction energy near the basis set limit and the CCSD(T) correction term using the 6-311G* basis set. The calculated CCSD(T) level energy profile has shown that the potential is very flat and the interconversion barrier height is very small (around 0.2 kcal/mol). The MP2 calculations using large basis sets near the basis set limit considerably overestimate the attraction of the slipped-parallel dimer, which indicates the importance of higher level electron correlation correction for studying the potential energy surface of the benzene dimer.