Julia E. Rice

Problems in the comparison of theoretical and experimental hyperpolarizabilities

Frequently it is useful to compare experimental values of the hyperpolarizabilities β and γ with calculated values. It is also often helpful to compare experimental values of β obtained from dc‐electric field induced second harmonic generation (dc‐SHG) experiments, e.g., with values obtained using the solvatochromism method. In order to do this the hyperpolarizabilities must be defined using consistent conventions. In this paper, four commonly used conventions are discussed and simple factors for converting between them presented. In addition, the sum‐over‐states expression for the calculation of β and γ is described and its correct use in comparing with hyperpolarizabilities obtained using other experimental and theoretical techniques discussed. As an illustration of the consistent use of conventions, ab initio and semiempirical calculations on para‐nitroaniline are compared with experimental dc‐SHG values. This comparison highlights the difference between theoretical values of the hyperpolarizability wi...

Analytic evaluation of energy gradients for the single and double excitation coupled cluster (CCSD) wave function: Theory and application

The theory for the analytic evaluation of energy gradients for coupled cluster (CC) wave functions is presented. In particular, explicit expressions for the analytic energy gradient of the CC singles and doubles (CCSD) wave function for a closed‐shell restricted Hartree–Fock reference determinant are presented and shown to scale as N6 where N is the one‐electron number of atomic basis functions for the molecular system. Thus analytic CCSD gradients are found to be of the same magnitude in computational cost as is the evaluation of analytic gradients for the configuration interaction singles and doubles (CISD) wave function. Applications of this method are presented for the water molecule and the formaldehyde molecule using a double‐ζ plus polarization (DZ+P) basis set. The CCSD equilibrium geometries, dipole moments, and, via finite differences of gradients, CCSD harmonic vibrational frequencies and infrared intensities are reported. For H2O these results are compared to analogous CISD, CISDT, CISDTQ, and...

DiscoveryLink: a system for integrated access to life sciences data sources

Vast amounts of life sciences data reside today in specialized data sources, with specialized query processing capabilities. Data from one source often must be combined with data from other sources to give users the information they desire. There are database middleware systems that extract data from multiple sources in response to a single query. IBMs DiscoveryLink is one such system, targeted to applications from the life sciences industry. DiscoveryLink provides users with a virtual database to which they can pose arbitrarily complex queries, even though the actual data needed to answer the query may originate from several different sources, and none of those sources, by itself, is capable of answering the query. We describe the DiscoveryLink offering, focusing on two key elements, the wrapper architecture and the query optimizer, and illustrate how it can be used to integrate the access to life sciences data from heterogeneous data sources.

The closed‐shell coupled cluster single and double excitation (CCSD) model for the description of electron correlation. A comparison with configuration interaction (CISD) results

A single and double excitation coupled cluster (CCSD) method restricted to closed‐shell single configuration reference functions is described in explicit detail. Some significant simplifications resulting from the restriction to closed‐shell systems are exploited to achieve maximum computational efficiency. Comparisons for energetic results and computational requirements are made with the single and double excitation configuration interaction (CISD) method. The specific molecules considered include N2, H2O, H3O+, H5O+2, HSOH, and s‐tetrazine (C2N4H2).

Solvent dependence of the second order hyperpolarizability in p-nitroaniline

Abstract Values of the second order hyperpolarizability β have been determined for p-nitroaniline (PNA) in solvents of varying polarity using the electric field induced second harmonic generation experimental technique. An observed solvent dependence of β0, the second order hyperpolarizability extrapolated to zero-frequency, is correlated with the solvent-induced shift of the PNA charge transfer absorption maximum λmax. The correlation is similar to the correlation between λmax and β0 observed in donor/acceptor p-substituted benzenes with different substituents. Both dependences can be understood within the framework of a simple four level Huckel model.

The analytic configuration interaction gradient method: Application to the cyclic and open isomers of the S3 molecule

The theory for the ab initio evaluation of potential energy gradients at the configuration interaction level of accuracy is presented, with special attention to the treatment of the various types of orbitals (frozen core, active, frozen virtual) which may arise. The new method has been used for a study of the D3h and C2v geometries of S3. SCF and CISD calculations predict the D3h structure to be lower than the C2v by 9.3 and 2.3 kcal/mol, respectively, whereas CASSCF and MRCISD predict the C2v structure to be the lowest by 8.9 and 8.2 kcal/mol, respectively, using good basis sets. These calculations support the prediction of Carlsen and Schaefer that both forms should be observable experimentally.

The elimination of singularities in derivative calculations

Abstract The evaluation of gradients and second derivatives of the electronic energy of a molecule is discussed for two methods which include electron correlation effects - configuration interaction and perturbation theory. It is shown that numerically stable procedures can be devised using the fact that the energy is invariant to certain orbital rotations amongst occupied and amongst virtual orbitals. Some details are given on the implementation of this procedure for closed-shell second-order perturbation theory second derivatives.

Theoretical investigations of molecules composed only of fluorine, oxygen and nitrogen: determination of the equilibrium structures of FOOF, (NO)2 and FNNF and the transition state structure for FNNF cis-trans isomerization

The deficiencies of common ab initio methods for the reliable prediction of the equilibrium structures of compounds composed of only the fluorine, oxygen and nitrogen atoms are investigated. Specifically, the importance of using large one-particle basis sets with multiple sets of polarization functions has been studied. Additionally, the need for a set of f basis functions was investigated. Several different single reference electron correlation methods have been tested in order to determine whether it is possible for a single reference based method to be routinely used on such chemical systems. These electron correlation methods include second order Møller-Plesset perturbation theory (MP2), singles and doubles configuration interaction (CISD), the coupled pair functional (CPF) approach and singles and doubles coupled cluster (CCSD) theory. The molecular systems studied include difluoroperoxide (FOOF), the cis form of the NO dimer, cis and trans difluorodiazene (FNNF) and the transition state to interconversion of the cis and trans isomers of FNNF. To the best of our knowledge, this is the first time that the cis-trans isomerization transition state has been reported. At the highest level of theory employed, the equilibrium structures of cis and trans FNNF agree very well with the experimental structures. However, the barrier to interconversion is predicted to be 65 kcal/mole, which is substantially higher than the experimental activation energy of 32 kcal/mole. Potential sources of error are discussed. A new diagnostic method for determining a priori the reliability of single reference based electron correlation methods is suggested and discussed.

The Reaction Mechanism for the Organocatalytic Ring-Opening Polymerization of l-Lactide Using a Guanidine-Based Catalyst: Hydrogen-Bonded or Covalently Bound?

We have investigated two alternative mechanisms for the ring-opening polymerization of l-lactide using a guanidine-based catalyst, the first involving acetyl transfer to the catalyst, and the second involving only hydrogen bonding to the catalyst. Using computational chemistry methods, we show that the hydrogen bonding pathway is considerably preferred over the acetyl transfer pathway and that this is consistent with experimental information.

THE CALCULATION OF FREQUENCY-DEPENDENT POLARIZABILITIES AS PSEUDO-ENERGY DERIVATIVES

The definition of frequency‐dependent polarizabilities α(−ω;ω), β(−2ω;ω,ω), β(−ω;ω,0), and β(0;ω,−ω) is discussed, and it is argued that the most convenient definitions are as energy derivatives, a pseudo‐energy being defined as the expectation value of [H−i(∂/∂t)]. This definition outlines a straightforward procedure for obtaining frequency‐dependent polarizabilities for all quantum chemistry methods including those which account for the effects of electron correlation. It is demonstrated at the self‐consistent field level of theory that αλμ(−ω;ω) cos ωt may be considered as the derivative of the static dipole moment μλ with respect to the strength Eωμ of a frequency‐dependent field Eωμ cos ωt (as is usual), or as the derivative of an appropriately defined frequency‐dependent dipole moment μμ cos ωt with respect to a static field E0λ. In this way, polarizabilities may be determined from finite static field calculations on lower‐order tensors. Therefore, α(−ω;ω) cos ωt is defined within second‐order Mo/ll...