Effective local potentials for excited states

The constrained variational Hartree-Fock method for excited states of the same symmetry as the ground state [Chem. Phys. Lett. 287, 189 (1998)] is combined with the effective local potential (ELP) method [J. Chem. Phys. 125, 081104 (2006)] to generate Kohn-Sham-type exact-exchange potentials for singly excited states of many-electron systems. Illustrative examples include the three lowest (2)S states of the Li and Na atoms and the three lowest (3)S states of He and Be. For the systems studied, excited-state ELPs differ from the corresponding ground-state potentials in two respects: They are less negative and have small additional "bumps" in the outer electron region. The technique is general and can be used to approximate excited-state exchange-correlation potentials for other orbital-dependent functionals.     

Effective potentials for spectator groups in molecular systems I. Potential curves and binding energies

Summary A method for representing inactive groups, i.e. spectator groups, in a molecular system by an effective potential is presented. The matrix elements for the spectator's short-range Hartree-Fock potential is stored in an intermediate AO basis, from which it can be transferred into the user basis for the active part of the molecular system. The longer-range of the potential is transferred via a (distributed) multipole expansion. The method is illustrated for the NH₃·X (X=NH₃, H₂O, HF) complexes: binding energies could be reproduced to within 5% by employing the effective NH₃ potential (whereby the lone pair was included in the active system), the entire NH₃·HF potential curve with a depth of 50 kJ/mol is reproduced within 2 kJ/mol if various intermediate basis sets are chosen. Technical details are discussed; the effective potential can directly be introduced in CI calculations.     

Effective local potentials for orbital-dependent density functionals

Practicality of the Kohn-Sham density functional scheme for orbital-dependent functionals hinges on the availability of an efficient procedure for constructing local exchange-correlation potentials in finite basis sets. We have shown recently that the optimized effective potential (OEP) method, commonly used for this purpose, is not free from difficulties. Here we propose a robust alternative to OEPs, termed effective local potentials (ELPs), based on minimizing the variance of the difference between a given nonlocal potential and its desired local counterpart. The ELP method is applied to the exact-exchange-only problem and shown to be promising for overcoming troubles with OEPs.     

Hard sphere perturbation theory for fluids with soft-repulsive-core potentials

The thermodynamic properties of fluids with very soft repulsive-core potentials, resembling those of some liquid metals, are predicted with unprecedented accuracy using a new first-order thermodynamic perturbation theory. This theory is an extension of Mansoori-Canfield/Rasaiah-Stell (MCRS) perturbation theory, obtained by including a configuration integral correction recently identified by Mon, who evaluated it by computer simulation. In this work we derive an analytic expression for Mon's correction in terms of the radial distribution function of the soft-core fluid, g(0)(r), approximated using Lado's self-consistent extension of Weeks-Chandler-Andersen (WCA) theory. Comparisons with WCA and MCRS predictions show that our new extended-MCRS theory outperforms other first-order theories when applied to fluids with very soft inverse-power potentials (n< or =6), and predicts free energies that are within 0.3 kT of simulation results up to the fluid freezing point.     

A method for molecular ionization potentials

A new method for one-electron propagator calculations of molecular inization potentials is proposed, using a large matrix technique The results of some trial calculations on molecular nitrogen are given.     

Bulk modulus for some simple molecular fluids

The bulk modulus B of several molecular fluids composed of rigid molecules has been calculated from p-p-T data obtained with a high-pressure vibrating tube densitometer. The data of all the substances studied, including Ar, can be described by a single master curve when plotted versus the reduced density *, in agreement with the predictions of the Gubbins-Gray perturbation theory for fluids with the same reference system. Combination of p-p-T and C_v data with the virial theorem has allowed the calculation of the exponent characterizing the repulsive branch of the intermolecular potential n. The different values of n suggest that different reference systems should be used for each substance, in contradiction with the conclusions obtained from the B versus * curves. This indicates that p-p-T data are less sensitive to the details of the intermolecular potential than their combination with other thermal properties like C_v, internal energy or residual entropy.     

Optimized theory for simple and molecular fluids

An optimized closure approximation for both simple and molecular fluids is presented. A smooth interpolation between Perkus-Yevick and hypernetted chain closures is optimized by minimizing the free energy self-consistently with respect to the interpolation parameter(s). The molecular version is derived from a refinement of the method for simple fluids. In doing so, a method is proposed which appropriately couples an optimized closure with the variant of the diagrammatically proper integral equation recently introduced by this laboratory [K. M. Dyer et al., J. Chem. Phys. 123, 204512 (2005)]. The simplicity of the expressions involved in this proposed theory has allowed the authors to obtain an analytic expression for the approximate excess chemical potential. This is shown to be an efficient tool to estimate, from first principles, the numerical value of the interpolation parameters defining the aforementioned closure. As a preliminary test, representative models for simple fluids and homonuclear diatomic Lennard-Jones fluids were analyzed, obtaining site-site correlation functions in excellent agreement with simulation data.     

An optimized algebraic basis for molecular potentials

The computation of vibrational spectra of diatomic molecules through the exact diagonalization of algebraically determined matrices based on powers of Morse coordinates is made substantially more efficient by choosing a properly adapted quantum mechanical basis, specifically tuned to the molecular potential. A substantial improvement is achieved while still retaining the full advantage of the simplicity and numerical light-weightedness of an algebraic approach. In the scheme we propose, the basis is parametrized by two quantities which can be adjusted to best suit the molecular potential through a simple minimization procedure.     

An extended rism equation for molecular polar fluids

The RISM integral equation is extended to molecules with charged sites via a renormalization of the Coulomb potentials and the introduction of appropriate closure relations. For a fluid of diatomics with atomic charges of ±0.2 e the equation yields site-site correlation functions in qualitative agreement with those from computer simulation.     

Evaluation of site-site bridge diagrams for molecular fluids

The presence of bridge functions in formally exact integral equation theories is the primary obstacle preventing the extraction of exact fluid structure from these theories. The bridge functions are typically neglected but in many fluids their impact may be significant. Each bridge function can be subdivided into bridge diagrams, which are well defined but difficult to evaluate. The calculation of bridge diagrams for the Chandler-Silbey-Ladanyi (CSL) integral equation theory is the subject of this paper. In particular, we evaluate the diagrams required to yield an exact theory up to the first power in density [O(rho(1))] and provide algorithms that remain feasible for any molecule. Further, the bridge diagrams are evaluated and compared with the f-bond and h-bond formulations. Exact bridge diagrams are numerically evaluated for several chiral molecules, for two polar dimers, and for SPC/E water. The quality of the diagrams is assessed in two ways: First, the predicted interatomic distributions are compared with those obtained from Monte Carlo simulations. Second, the connectivity constraints are evaluated and the errors in satisfying these exact relationships are compared for the f-bond and h-bond formulations. For apolar fluids, a clear improvement in CSL theory is evident with the inclusion of O(rho(0)) and O(rho(1)) diagrams. In contrast, for polar fluids, the inclusion of bridge diagrams does not lead to improvement in the structural predictions.     

Supercritical fluids as reaction media for molecular catalysis

Chemical transformations in supercritical fluids (SCFs) have enormous potential advantages. The possibilities of rate enhancement and adjustable selectivities will motivate the research of molecular catalysis in SCFs. Recent progress in the organometallic catalysis under supercritical conditions is reviewed with emphasis on the benefits of utilization of supercritical carbon dioxide (scCO₂) both as a reaction medium and reactant.     

Effective potentials between nanoparticles in suspension

Results of molecular dynamics simulations are presented for the pair distribution function between nanoparticles in an explicit solvent as a function of nanoparticle diameter and interaction strength between the nanoparticle and solvent. The effect of including the solvent explicitly is demonstrated by comparing the pair distribution function of nanoparticles to that in an implicit solvent. The nanoparticles are modeled as a uniform distribution of Lennard-Jones particles, while the solvent is represented by standard Lennard-Jones particles. The diameter of the nanoparticle is varied from 10 to 25 times that of the solvent for a range of nanoparticle volume fractions. As the strength of the interactions between nanoparticles and the solvent increases, the solvent layer surrounding the nanoparticle is formed which increases the effective radii of the nanoparticles. The pair distribution functions are inverted using the Ornstein-Zernike integral equation to determine an effective pair potential between the nanoparticles mediated by the introduction of an explicit solvent.     

Transient molecular dynamics simulations of viscosity for simple fluids

A transient molecular dynamics (TMD) method has been developed for simulation of fluid viscosity. In this method a sinusoidal velocity profile is instantaneously overlaid onto equilibrated molecular velocities, and the subsequent decay of that velocity profile is observed. The viscosity is obtained by matching in a least-squares sense the analytical solution of the corresponding momentum transport boundary-value problem to the simulated decay of the initial velocity profile. The method was benchmarked by comparing results obtained from the TMD method for a Lennard-Jones fluid with those previously obtained using equilibrium molecular dynamics (EMD) simulations. Two different constitutive models were used in the macroscopic equations to relate the shear rate to the stress. Results using a Newtonian fluid model agree with EMD results at moderate densities but exhibit an increasingly positive error with increasing density at high densities. With the initial velocity profiles used in this study, simulated transient velocities displayed clear viscoelastic behavior at dimensionless densities above 0.7. However, the use of a linear viscoelastic model reproduces the simulated transient velocity behavior well and removes the high-density bias observed in the results obtained under the assumption of Newtonian behavior. The viscosity values obtained using the viscoelastic model are in excellent agreement with the EMD results over virtually the entire fluid domain. For simplicity, the Newtonian fluid model can be used at lower densities and the viscoelastic model at higher densities; the two models give equivalent results at intermediate densities.     

Ab initio molecular electrostatic potentials

The ab initio isopotential map of guanine is given and compared to that of adenine. It shows that in contrast to the situation in adenine, the most basic site of guanine is N₇ with a secondary potential minimum at O₆. These results as well as those concerning the secondary out-of-plane attractive regions over the NH₂ group and C₈ H bonds of the two molecules are discussed in connection with the available experimental knowledge concerning the bonding of alkylating carcinogens and mutagens.     

Moletronics modeling toward molecular potentials

Using ab initio density functional theory (DFT) calculations, we demonstrate two molecular OR gates that are able to process binary signals encoded as molecular potentials. Thus, the possibility to implement logic gates of <1 nm is demonstrated. The advantage of this approach to post‐microelectronics technologies is the tremendous low‐power dissipation, the small feature size of molecular devices, and the compatible nature of input and output signals that would allow the implementation of complex logic. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

镧系金属一氢化物有效芯势研究

采用Cundari和Stevens等推导的有效芯势对镧系金属一氢化物进行了理论计算，以探讨镧系金属元素与氢的相互作用。结果表明所有镧系金属一氢化物基态时理论上是稳定的，最稳定的是SmH,最不稳定的是DyH;键长计算结果显示，基态时镧系金属一招兵买马花物有独立王国 收缩现象发生；红外振动频率理论计算值与实验结果一致；成键轨道中，金属原子轨道的贡献主要是s轨道和d轨道：从CeH至ErH(GdH)例外）随着外层电子的增加s轨道成分逐渐增大d轨道成分逐渐减小；从TmH和LuH（包括GdH)，成键轨道中金属原子轨道的贡献主要是d轨道，约为90％；约大多数镧系金属一氧化物的成键轨道中金属原子轨道f成分小于1％。     

Self-assembly of T-structures in molecular fluids

A lattice density functional approach is used to describe the equilibrium assembly of three types of anisotropic patchy particles into a T-structure. The T-structure is comprised of one three-patch, three two-patch, and three one-patch particles. All patches are positioned orthogonal to each other. Temperature, particle concentration, and interaction energy ranges are determined that lead to T-structure formation. T-structure formation is investigated for two types of two-patch particles: Case 1 uses two identical patches and Case 2 employs two differing patches. Sets of parameters leading to T-structure assembly are determined for both cases. We find that in Case 1 the symmetric two-patch particle enforces T-structure formation, while the asymmetric two-patch particle in Case 2 leads to formation of chains, dimers, and incorrect and extended T-structures in addition to the T-structure. Synthetic strategies for both cases are discussed and reveal that Case 2 presents the more straightforward synthetic route.     

Quantitative comparison of molecular electrostatic potentials for structure-activity studies

An approach for representing, efficiently calculating and comparing discrete three-dimensional molecular electrostatic potentials using a quantitative similarity index (MEP-SI) based on a Carbo-type formalism is presented. A radial-type (MACRA) grid representation is described that provides more efficient storage of MEP information than a cubic grid of similar range, appropriate emphasis, and a convenient means for restricting the comparison of MEP functions to a local molecule region. The MACRA based MEP-SI formalism was used to evaluate the suitability of a variety of approximate methods for efficiently calculating the MEP for use in MEP-SI comparison of dissimilar molecules. The Mulliken charge method was found inadequate, while the method of potential-derived charges (PDCs), with additional charges for lone electron pairs included on sulfur, provided an efficient and sufficiently accurate representation of the MEP for this purpose. Convergence of the MEP-SI with respect to MACRA grid extent and mesh size was demonstrated; the effect of MEP error and different grid point emphasis in the MACRA versus the cubic grid results was investigated, and MEP-SI results were compared for different forms of the SI equation. The methodology proposed in this study provides an efficient and practical means for comparing MEP functions for two molecules and gives discriminating results for a sample series of molecular analogues consistent with expectations.