Restricted Hartree-Fock Formalism Research Articles

Theoretical and experimental studies of ion imprinted polymer for nitrate detection

Molecular imprinting is an approach to synthesize receptors with specific molecular recognition properties. A computational method was carried out to study interaction between template and monomer in prepolymerization mixture. The functional monomer and template complexes were optimized, at the minimum energy confirmation using Austin Model 1 semi empirical method within Restricted Hartree Fock formalism. The theoretical results showed that allylthiourea (functional monomer) has the largest interaction energy towards template (sodium nitrate) with the mole ratio of 4 : 1; functional monomer : template. The resulting polymers were characterized using Fourier Transform infrared spectroscopy, thermogravimetry analysis and field emission scanning electron microscopy. Rebinding experiments were carried out to evaluate binding capacity of the polymer. The adsorption data of ion imprinted polymer (IIP) were fitted with Langmuir-Freundlich isotherm model. Pseudo-second order kinetic model was used to describe the kinetic adsorption behavior of IIP. The experimental binding result showed good agreement with theoretical computation and the IIP was further used for nitrate ion detection. The results of membrane optimization indicated that the sensor, which composed of 30% polyvinylchloride, 60% nitrophenyl octyl ether as a plasticizer, 2% sodium tetraphenyl borate, and 10% IIP as ionophore exhibited an almost Nernstian slope with the limit of detection 3.9 × 10-6 M. The fabricated sensor had shown good potential in nitrate detection with wide linear range, low limit of detection and found to have good selectivity towards nitrate ion over other anion.

An attempt to decompose the force constants for some diatomic molecules by the derivatives of the electronic kinetic energy

The force constants for several diatomic molecules were calculated by the derivatives of the electronic kinetic energy within the restricted Hartree–Fock formalism. The uniform scaling procedure was utilized in order to satisfy the virial theorem. The decomposition of the force constant was performed by partitioning the derivatives of the kinetic energy in several ways.

Computational methods for the study of enzymic reaction mechanisms III: A perturbation plus QM/MM approach for calculating relative free energies of protonation

We describe a coupling parameter, that is, perturbation, approach to effectively create and annihilate atoms in the quantum mechanical Hamiltonian within the closed shell restricted Hartree-Fock formalism. This perturbed quantum mechanical atom (PQA) method is combined with molecular mechanics (MM) methods (PQA/MM) within a molecular dynamics simulation, to model the protein environment (MM region) effects that also make a contribution to the overall free energy change. Using the semiempirical PM3 method to model the QM region, the application of this PQA/MM method is illustrated by calculation of the relative protonation free energy of the conserved OD2 (Asp27) and the N5 (dihydrofolate) proton acceptor sites in the active site of Escherichia coli dihydrofolate reductase (DHFR) with the bound nicotinamide adenine dinucleotide phosphate (NADPH) cofactor. For a number of choices for the QM region, the relative protonation free energy was calculated as the sum of contributions from the QM region and the interaction between the QM and MM regions via the thermodynamic integration (TI) method. The results demonstrate the importance of including the whole substrate molecule in the QM region, and the overall protein (MM) environment in determining the relative stabilities of protonation sites in the enzyme active site. The PQA/MM free energies obtained by TI were also compared with those estimated by a less computationally demanding nonperturbative method based on the linear response approximation (LRA). For some choices of QM region, the total free energies calculated using the LRA method were in very close agreement with the PQA/MM values. However, the QM and QM/MM component free energies were found to differ significantly between the two methods.

The strong dimer of Ge 60, Ge 116

The strong dimer of Ge 60, Ge 116, is considered for MNDOd and AM1 type semiempirical quantum chemical calculations within the limitations of restricted Hartree–Fock formalism. Both methods predict this, yet non-existent, structure to be stable but highly endothermic. Some geometrical and quantum chemical data are presented.

High pressure Raman studies on the structural conformation of oligophenyls

The goal of this combined experimental and computational study is to investigate the structural conformation of oligo( para-phenylenes) in the crystalline phase, in particular the planarity of this type of molecules. To this end we have performed Raman experiments on para-terphenyl and para-quaterphenyl in a pressure range from 0 to 70 kbar and at temperatures from 10 to 300 K. The positions and the relative intensities of the C–C interring stretch mode at 1280 cm −1 and the C–H in-plane bend mode at 1220 cm −1 have been tracked. We find that upon increasing temperature at ambient pressure the intensity ratio I 1280/ I 1220 drops rapidly at temperatures that coincide with the crystallographic phase transition for the investigated materials. At ambient temperature also, this intensity ratio drops rapidly upon increasing pressure up to about 15 kbar. In the computational part, the Raman frequencies and activities of isolated 3P and 4P molecules were calculated within restricted Hartree–Fock formalism with the interring tilt angles varying from 0 to 90°. These calculations confirm that the I 1280/ I 1220 intensity ratio can be related to the planarity of the molecules. Three-dimensional bandstructure calculations within density functional theory were applied to determine phonon frequencies and estimate Raman activities for the polymer poly( para-phenylene). These simulations show that the same conclusions hold for crystalline environment.

Restricted open shell Hartree-Fock theory

A new formula is introduced for the energy expectation value of multi-determinantal single configuration states with arbitrary open shells in terms of the 4-indexed electronic repulsion integrals. Applying the variational principle to this expression and introducing 2-indexed Coulomb and exchange operators, new forms of Hartree-Fock and Hartree-Fock-Roothaan equations for open shell systems are obtained. They are based entirely on the restricted Hartree-Fock formalism.

GaAs-based clusters.

This paper presents the results of cluster calculations performed for selected Ga- and As-based clusters: These include the GaH 3 , AsH 3 , GaAsH 6 , and Ga 2 As 2 H 10 model clusters. Ab initio calculations were performed within the restricted Hartree-Fock formalism. An effective core potential is adopted in order to limit these calculations to valence electrons. The total energies, equilibrium geometries, and harmonic vibrational frequencies are calculated for each of the model clusters. Binding energies for both the GaH 3 and AsH 3 model clusters are determined

Spin adapted restricted open shell coupled cluster theory. Linear version

We describe the linear version of the spin adapted coupled cluster theory with all single and double excitations for open shell systems in restricted Hartree–Fock formalism. The method is based on the partitioning of the Hamiltonian developed by Hubač and Čársky [I. Hubač and P. Čársky, Phys. Rev. A 22, 2392 (1980)] in restricted Roothaan open shell formalism. Correlation energy of some simple molecular systems is calculated.

Restricted Hartree-Fock and unrestricted Hartree-Fock as reference states in many-body perturbation theory: a critical comparison of the two approaches

The paper deals with two topics related to the problem which reference state is better for many-body perturbation theory: restricted Hartree-Fock (RHF) or unrestricted Hartree-Fock (UHF)? The first topic concerns the potential surfaces. Several examples are presented to show shortcomings of the two approaches and a simple way is presented which seems to give a useful potential curve in the whole range of interatomic distances by a composition of RHF and UHF potential curves. The second topic concerns the many-body perturbation theory for open-shell systems in the RHF formalism. The method is critically examined and compared with the ordinary many-body perturbation theory using UHF as the reference. This examination of many-body techniques provides also some insight into the problems inherent of the SCF theory: spin contamination from higher multiplets, localization of orbitals, and self-consistency effects.

AnAb Initio computational scheme for polymeric chains with fully converged coulomb and exchange lattice sums

The restricted Hartree–Fock formalism applied to quasi one-dimensional translational systems embodies slowly convergent Coulomb and exchange lattice summations. In this contribution, an algorithm based on a Filon like quadrature procedure to carry out the k-space integration of density matrix elements is analyzed and its efficiency is illustrated by its application to the linear chains of hydrogen molecules. It allows the computation of Coulomb and exchange lattice sums to their asymptotic limit, and renders obsolete the empirical procedure of guessing the number of interactions to be included in the calculations.

Ab initio scf studies on thioformic acid: basis-set effects

Ab initio calculations on monothioformic acid (HCOSH, TFA) were performed using the restricted Hartree-Fock formalism and 13 standard Gaussian basis sets in order to determine the basis-set dependence of the geometries, energy barriers and vibrational spectra of cis- and trans-TFA. Trans-TFA was found to be the more stable with the trans-cis energy difference varying between 5.0 and 13.8 kJ mol −1and the trans-cis energy barrier height ranging from 31.2 to 42.1 kJ mol −1. The inclusion of polarization functions in the basis set unproved considerably the calculated values compared with the available experimental data. Furthermore, the addition of diffuse functions did not noticeably influence the results. A normal-coordinate analysis based on HF/6–311G ∗∗ calculated Cartesian force field was performed to describe the assignment of the fundamental vibrational frequencies.

AB initio molecular orbital studies on 1-fluoropropane

Geometries for both gauche and trans conformers of 1 -fluoropropane have been optimized using restricted Hartree-Fock formalism with 4-31G and 6-31G ** Gaussian basis sets. The asymmetric torsional potential energy profile for rotation of the CH 2F group has been computed at the HF/ 4-31G level and expanded as a truncated Fourier cosine series. Additional single point calculations have been carried out for both minima and saddle points at the HF/6-31G **, MP2(FULL)/431G//HF/4-31G and MP2(FULL)/6-31G **//HF/6-31G ** levels. Regardless of the level of theory used in calculation, gauche appears to be the more stable conformer with the trans-gauche energy difference varying from 1.08 to 2.21 kJ mol −1. According to the calculations the gauche → trans torsional barrier is estimated to be between 17.0 and 19.2 kJ mol −1 and the barrier separating the equivalent gauche forms between 18.6 and 20.3 kJ mol −1. A vibrational analysis based on ab initio 4-31G calculated force field is given for both conformers and compared with previous matrix isolation results.

Positron and electron energy bands in several ionic crystals using restricted Hartree-Fock method

Using a restricted Hartree-Fock formalism and suitably localized and symmetrized wave functions, both the positron and electron energy bands were calculated for NaF, MgO and NiO. The lowest positron state at Γ 1 lies above the vacuum level and negative work functions are predicted. Positron annihilation rates were calculated and found to be in good agreement with measured lifetimes.

Electronic Structure of the First Excited State of CO. I. SCF Wavefunction Calculated in the Restricted Hartree—Fock Formalism

An SCF wavefunction calculated in the restricted Hartree—Fock formalism is reported for the lowest excited state (a 3II) of CO. The expansion method is used and exponents are optimized. Even though the calculated energy and Hellmann—Feynman forces are reasonable, there is a large discrepancy between the calculated and experimental dipole moment. The discrepancy is caused by the fact that the restricted Hartree—Fock function for open-shell systems does not satisfy Brillouin's theorem and first-order corrections come in. In particular, it is found that the charge-transfer states have a large first-order effect on the dipole moment. Some preliminary results on the isoconfigurational A 1II state are reported. They indicate that the charge distributions of the two isoconfigurational states are quite different.

Restricted and unrestricted hartree-fock calculations for atomic lithium

The unsymmetrized unrestricted and several restricted versions of the Hartree-Fock formalism are used in calculations for atomic lithium. The calculations lead to very similar results. This suggests that it is advantageous to use a particularly tractable version of the restricted Hartree-Fock formalism.