A selected ion flow tube study of the ion-molecule reactions of monochloroethene, trichloroethene and tetrachloroethene

Data for the rate coefficients and product cations of the reactions of a large number of atomic and small molecular cations with monochloroethene, trichloroethene and tetrachloroethene in a selected ion flow tube at 298 K are reported. The recombination energy of the ions range from 6.27 eV (H$_3$O$^+$) through to 21.56 eV (Ne$^+$). Collisional rate coefficients are calculated by modified average dipole orientation theory and compared with experimental values. Thermochemistry and mass balance predict the most feasible neutral products. Together with previously reported results for the three isomers of dichloroethene (J. Phys. Chem. A., 2006, 110, 5760), the fragment ion branching ratios have been compared with those from threshold photoelectron photoion coincidence spectroscopy over the photon energy range 9-22 eV to determine the importance or otherwise of long-range charge transfer. For ions with recombination energy in excess of the ionisation energy of the chloroethene, charge transfer is energetically allowed. The similarity of the branching ratios from the two experiments suggest that long-range charge transfer is dominant. For ions with recombination energy less than the ionisation energy, charge transfer is not allowed; chemical reaction can only occur following formation of an ion-molecule complex, where steric effects are more significant. The products that are now formed and their percentage yield is a complex interplay between the number and position of the chlorine atoms with respect to the C=C bond, where inductive and conjugation effects can be important

Correct quantum chemistry in a minimal basis from effective Hamiltonians

We describe how to create ab-initio effective Hamiltonians that qualitatively describe correct chemistry even when used with a minimal basis. The Hamiltonians are obtained by folding correlation down from a large parent basis into a small, or minimal, target basis, using the machinery of canonical transformations. We demonstrate the quality of these effective Hamiltonians to correctly capture a wide range of excited states in water, nitrogen, and ethylene, and to describe ground and excited state bond-breaking in nitrogen and the chromium dimer, all in small or minimal basis sets

Higher Energy Derivatives in Hilbert Space Multi-Reference Coupled Cluster Theory : A Constrained Variational Approach

In this paper, we present formulation based on constrained variational approach to compute higher energy derivatives upto third order in Hilbert Space Multi-Reference Coupled Cluster (HSMRCC) Theory. This is done through the use of a functional with Lagrange multipliers corresponding to HSMRCC method, as done by Helgaker, Jorgensen and Szalay. We derive explicit expressions upto third order energy derivatives. Using (2n + 1) and (2n + 2) rules, the cancellation of higher order derivatives of functional parameters that are not necessary according to these rules, is explicitly demonstated. Simplified expressions are presented. We discuss several aspects of the functional used and its potential implications

Development of an efficient linear response approach to the Hilbert space multi-reference coupled-cluster theory

In this paper, we use an analytic linear response to develop efficient expressions for calculating a first-order energy response using the multi-reference Hilbert space coupled-cluster (HSMRCC) theory. Equations for the first-order response are derived and their diagrammatic evaluation is outlined. The Z-vector formalism used in SRCC to eliminate the explicit presence of a cluster amplitude response in favor of a de-excitation operator is generalized to HSMRCC and applied here. We also discuss several aspects of the Z-vector and outline different ways of introducing the technique and appropriateness of these in various circumstances. Efficient expressions for the energy response in terms of state-dependent effective CC density matrices are presented. We also compare our approach with Szalay's approach based on the generalized Hellmann-Feynmann theorem and discuss the advantages of our approach

Potential Energy Curves of Molecular Nitrogen for Singly and Doubly Ionized States with Core and Valence Holes

Our manuscript, presents the computation of potential energy curves of all possible singly and doubly ionized states of molecular nitrogen. Accurate representation of the potential energy curves of ionized states of N2 is essential to explicitly treat coupled electron-nuclear dynamics. In this work, we compute the potential energy curves of the valence as well as the core and inner valence singly and doubly ionized states of N2. These curves pave way to study the interplay between photoionization and Auger spectra when molecular nitrogen interacts with free electron lasers.Computation of inner valence or core ionized potential energy curve is not trivial due to the well-known problem of variational collapse of the wavefunction to the lowest energy state. We circumvent this problem by implementing a two-step optimization scheme within the multi-configurational self-consistent field approach. Such a two-step optimization scheme has been previously implemented to compute potential energy curves of core ionized states of di-atomic molecules with one hole. Herein, we show the general applicability of this two-step optimization method by computing potential energy curves of both singly and doubly ionized states of N2 with valence and core holes. Calculation of potential energy curves for core ionized polyatomic systems are scarce. Moreover, our approach is system independent and can be easily extended to calculate multiple-core ionized states. To the best of our knowledge, this is the first calculation of potential energy curves for doubly ionized states of a diatomic molecule with two core (or inner valence) holes.</p

Reflections on size-extensivity, size-consistency and generalized extensivity in many-body theory

An overview is presented of the notions of size-extensivity and size-consistency, which play an important role in the discussion of many-body methods in physics and chemistry. We also introduce the concept of generalized extensivity, which has been used before implicitly in the literature, and provide an operational definition, which can be tested numerically in a similar way as size-consistency. A numerical example illustrates the concept of generalized extensivity as applied to the EOM-CCSD, STEOM-CCSD and extended-STEOM-CCSD electronic structure methods for excited states. In another line of thought it is argued that algebraic proofs are often more easily constructed than diagrammatic proofs to demonstrate proper separation into non-interacting fragments. The algebraic line of reasoning is illustrated for single reference methods and applied to discuss separability properties for a variety of Hilbert space multi-reference coupled cluster methods

A constrained variational approach for energy derivatives in Fock-space multireference coupled-cluster theory

In this paper, we present a formulation based on constrained variational approach to enable efficient computation of energy derivatives using Fock-space multireference coupled-cluster theory. Adopting conventional normal ordered exponential with Bloch projection approach, we present a method of deriving equations when general incomplete model spaces are used. Essential simplifications arise when effective Hamiltonian definition becomes explicit as in the case of complete model spaces or some special quasicomplete model spaces. We apply the method to derive explicit generic expressions upto third-order energy derivatives for [0,1], [1,0], and [1,1] Fock-space sectors. Specific diagrammatic expressions for zeroth-order Lagrange multiplier equations for [0,1], [1,0], and [1,1] sectors are presented