Fock space multireference coupled cluster theory: Study of shape resonance

The complex absorbing potential along with correlated independent particle potential (CIP) Fock space multireference coupled cluster method is used for the study of resonances. We have studied shape resonance of e^?‐ F₂, e^?‐ N₂O and e^?‐CO molecules. In particular, we have studied e^?‐ F₂ scattering at different bond lengths to know whether is bound at the equilibrium bond length of F₂. © 2013 Wiley Periodicals, Inc.     

A holomorphic representation approach to the regularization of model field theories in coupled cluster form

Summary We explain in detail the so-called Bargmann or holomorphic representation, and apply it to the general class of single-mode bosonic field theories. Since these model field theories have no attribute of separability and are, in some sense, maximally nonlocal, they are an especially severe test of the capability of coupled cluster methods to parametrize them satisfactorily. They include the cases of anharmonic oscillators of order 2K (K=2, 3,...), for which ordinary perturbation theory is known to diverge, and we therefore make a special study of such systems. We demonstrate for the first time for any quantum-mechanical problem with infinite Hilbert space that both the normal and extended coupled cluster methods (NCCM and ECCM) have phase spaces which rigorously exist. We analyze completely the asymptotic properties of the complete sets of the NCCM and ECCM amplitudes, either of which fully characterizes the system. It is thereby shown how the holomorphic representation can be used to regularize completely all otherwise formally divergent series that appear. We demonstrate in detail how the entire NCCM and ECCM programmes can be carried through for these systems, including the diagonalization of the classically mapped Hamilitonians in the respective classical NCCM and ECCM phase spaces.     

An overview of coupled cluster theory and its applications in physics

Summary What has since become known as the normal coupled cluster method (NCCM) was invented about thirty years ago to calculate ground-state energies of closed-shell atomic nuclei. Coupled cluster (CC) techniques have since been developed to calculate excited states, energies of open-shell systems, density matrices and hence other properties, sum rules, and the sub-sum-rules that follow from imbedding linear response theory within the NCCM. Further extensions deal both with systems at nonzero temperature and with general dynamical behaviour. More recently, a new version of CC theory, the so-called extended coupled cluster method (ECCM) has been introduced. It has the potential to describe such global phenomena as phase transitions, spontaneous symmetry breaking, states of topological excitation, and nonequilibrium behaviour. CC techniques are now widely recognized as providing one of the most universally applicable, most powerful, and most accurate of all microscopicab initio methods in quantum many-body theory. The number of successful applications within physics is now impressively large. In most such cases the numerical results are either the best or among the best available. A typical case is the electron gas, where the CC results for the correlation energy agree over the entire metallic density range to within less than 1 millihartree (or <1%) with the essentially exact Green's function Monte Carlo results. The role of CC theory within modern quantum many-body theory is first surveyed, by a comparison with other techniques. Its full range of applications in physics is then reviewed. These include problems in nuclear physics, both for finite nuclei and infinite nuclear matter; the electron gas; various integrable and nonintegrable models; various relativistic quantum field theories; and quantum spin chain and lattice models. Particular applications of the ECCM include the quantum hydrodynamics of a zero-temperature, strongly-interacting condensed Bose fluid; a charged impurity in a polarizable medium (e.g., positron annihilation in metals); and various anharmonic oscillator and spin systems.     

Employing broken symmetry effects from unrestricted coupled cluster wave function to determine dynamic and non-dynamic electron correlation during triple bond breaking in the N2 molecule

The cluster structure of the singlet full symmetric component of the unrestricted Hartree-Fock (UHF)-based CCSD wave function describing the triple bond breaking in the nitrogen molecule has been subjected to a detailed analysis for the possibility of determining the dynamic and near-degeneracy electron correlation. The results obtained show that the spin and symmetry contaminations are not responsible only for the appearance of the artificial hump in the potential energy curve (PEC) generated by the UHF-based CCSD calculations. A theoretical analysis of this issue indicates that the UHF-based CCSD wave function and its singlet full symmetric component have a multi-reference structure. This form of the wave function allows to explain the mechanism for creating cluster contributions in the projected UHF-based CCSD wave function, which also provides the opportunity to explain the cause of the hump in the nitrogen PEC generated by the UCCSDecCCSD method.     

The Fock space coupled cluster method: theory and application

Summary The Fock space coupled cluster method and its application to atomic and molecular systems are described. The importance of conserving size extensivity is demonstrated by the electron affinities of the alkali atoms. Two types of intruder states are discussed, one attributable to the orbital energy spectrum and the other caused by two-electron interactions. They are illustrated by the excited states of Li₂ and by¹ S states of Be, respectively. It is shown how both problems may be solved using incomplete model spaces. The selection of the model space in a moderately dense spectrum is discussed in connection with N₂ excited states.Supported in part by the U.S.-Israel Binational Science Foundation     

Comparison of some multireference electronic structure methods in illustrative applications

The performances of several multireference electronic structure methods including complete active space self-consistent field (CASSCF)-based second-order perturbation theory (CASPT2), multireference configuration interaction with single and double excitations (MR-CISD), MR-CISD with the Davidson correction (MR-CISD+Q), and the CASSCF-based block-correlated coupled cluster method (CAS-BCCC4) we developed recently are compared by applying them to study several different chemical problems involving computation...     

Multireference coupled cluster theory in Fock space

Summary A Fock space multireference coupled cluster method based on incomplete model spaces is described. Some of the essential computational aspects of the theory are discussed with the aid of the diagrammatic representation of the equations. An application to the calculation of ionization potentials and excitation energies ofs-tetrazine is presented along with comparisons with conventionalab initio calculations and experimental results.     

On the calculation of expectation values and transition matrix elements by coupled cluster method

Summary Finite order expressions are derived for expectation values and transition matrix elements within the framework of the coupled cluster method.     

Stationary coupled cluster response: Role of cubic terms in molecular properties

We have demonstrated an application of a stationary coupled cluster response approach for molecular properties using an Euler functional. This involves terms which are of cubic power in cluster amplitudes. We have shown that these are important terms and have also discussed the convergence properties of the functional for higher order properties.     

Direct evaluation of one-electron properties in coupled cluster methods

Summary An analysis of a method for approximate calculations of expectation values for one-electron operators from available coupled cluster amplitudes is presented and illustrated numerically for the polarizability of the Be atom. The one-particle density matrix resulting from the present approach is accurate through the fourth order in the electron correlation perturbation. It has been found that, in order to obtain quantitative agreement between the energy derivative results and the approximate expectation value formalism, the third orderT ₁ T ₂⁽⁰⁾ wave function term must be included into the calculation of the one-particle density matrix. The present method is also considered as a promising tool for calculations of higher-order atomic and molecular properties from high level correlated wave functions.     

基于CASSCF参考函数的块相关耦合簇方法对烷烃中单键解离势能面研究

用以完全活化空间自洽场(CASSCF)波函数为参考波函数的块相关耦合簇(BCCC)方法(简称CAS-BCCC)研究了烷烃(甲烷和乙烷)中的单键解离过程的势能面(PES). 与其它理论方法比较的结果表明, 该方法可以对所研究的整个解离势能面给出定量准确的描述.     

IMiCMO: a new integrated ab initio multicenter molecular orbitals method for molecular dynamics calculations in solvent cluster systems

A new computational scheme integrating ab initio multicenter molecular orbitals for determining forces of individual atoms in large cluster systems is presented. This method can be used to treat systems of many molecules, such as solvents by quantum mechanics. The geometry parameters obtained for three models of water clusters by the present method are compared with those obtained by the full ab initio MO method. The results agree very well. The scheme proposed in this article also intended for use in modeling cluster systems using parallel algorithms. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1107–1112, 2001     

微波消解-电感耦合等离子体原子发射光谱（ICP-AES）法测定镍基耐蚀合金中镍

采用微波消解法与电感耦合等离子体原子发射光谱(ICP-AES)法相结合的方式,建立了镍基耐蚀合金中镍元素含量的测定方法。试样在浓硝酸和浓盐酸中微波消解后,用电感耦合等离子体原子发射光谱仪在优化的工作参数下测定,结果表明,稀释的消解液可直接用于镍含量的测定,光谱干扰少。对镍基耐蚀合金中镍含量进行多次平行测定,相对标准偏差(RSD,n=7)为0.29%,且测得值与标准值结果一致。对不同的镍基耐蚀合金样品进行加标回收实验,加标回收率在98.5%~102%。标准物质验证实验表明,测得值与标准值一致。     

Magnesium interference and different efficiencies of diastereoisomeric cluster formation in phenylalanine enantiomeric discrimination by the kinetic method

The kinetic method has been applied for determination of d-Phe/l-Phe enantiomeric ratio. Discrimination of enantiomers was inferred from product ion mass spectra of trimeric cluster ions containing the analyte (l,d-Phe), Cu²⁺ as a central metal and l-Trp as a chiral reference ligand. Unsatisfactory quantitative results achieved on an ion trap were rationalized by high-resolution mass spectrometry. The formation of Mg²⁺-containing cluster isobaric to trimeric cluster [Cu(l-Trp)₂Phe]⁺ was observed. Interference like this was identified as a possible reason for deterioration of quantitative low-resolution mass spectrometric analyses of real-world samples based on the kinetic method. Cation-exchanger was used for easy removal of magnesium from a sample and improvement of quantitation.Chiral dependence of formation of the Cu²⁺-containing trimeric cluster was also observed. Heterochiral diastereoisomeric ions were created less effectively.     

Surface forces in foam films from ABA block copolymer: a dynamic method study

The thinning of foam films from aqueous solutions of an ABA triblock copolymer of polyethylene oxide and polypropylene oxide (average molecular weight 14,000 g/mol) is studied experimentally. The dependence of the surface forces on film thickness is obtained by the dynamic method of Scheludko and Exerowa.The total surface force measured in foam films (radius 60–70 m) from 10^–5 M (0.014 wt%) polymer solution with 0.1 M NaCl is positive at thicknesses from about 800 down to 460 . The electrostatic repulsion is negligible while the contribution of van der Waals attraction is small (within 15%). Therefore a positive surface force component predominates. Most probably it arises from steric interactions between the hydrophilic polyethylene oxide tails of the polymer. The dynamic method appears to be a suitable technique for exploring the stabilization of foam films from ABA copolymers.