A minimal model of potential energy surface for H2O?HCl

In this paper, we present a model of potential energy surface for the H₂O HCl system, consisting in the exact transformation of quantum chemical input data related to a minimal number of significant configurations. Both molecules are assumed as rigid. The interaction potential is given by an expansion in real spherical harmonics depending on the distance between the two centers of mass of the molecules and on four angles that define their mutual orientation. The main target of this work is the construction of a model of potential energy surface that requires a limited number of single energy points, which is suitable for applications to classical and quantum molecular dynamics simulations, permitting interpolation and further implementation of different sets of input data.     

How many dimensions are required to approximate the potential energy landscape of a model protein?

A scheme to approximate the multidimensional potential energy landscape in terms of a minimal number of degrees of freedom is proposed using a linear transformation of the original atomic Cartesian coordinates. For one particular off-lattice model protein the inherent frustration can only be reproduced satisfactorily when a relatively large number of coordinates are employed. However, when this frustration is removed in a Go-type model, the number of coordinates required is significantly lower, especially around the global potential energy minimum. To aid our interpretation of the results we consider modified disconnectivity graphs where a measure of the structural diversity and a metric relation between the stationary points are incorporated.     

Ab initio intermolecular potential energy surface of He-LiH

The intermolecular potential energy surface of He-LiH complex was studied using the full-electronic complete forth-order Miller-Plesset perturbation (MPPT) method.In ab initio calculations,the bond length of LiH was fixed at 0 159 5 nm.The potential has two local minima of Vm=-179.93 cm for the linear He LiH geormetrv at Rm=0.227 nm and Vm=-10.44 cm-1 for the linear He-HL1 geometry at Rm=0.516 nm The potemal exhibits strong anisotropy The analytic potential function with 31 parameters was determined by fitting to the calculated ab,mtio potentials The influence of variation of LiH bond length on the potential energy surface was also studied     

Theoretical studies on the potential energy surfaces and vibrational energy levels of HXeF and HXeCl

The potential energy surfaces for the electronic ground state of the HXeCl and HXeF molecules areconstructed by using the internally contracted multi-reference configuration interaction with theDavidson correction(icMRCI Q)method and large basis sets.The stabilities and dissociation barriersare identified from the potential energy surfaces.The three-body dissociation channel is found to bethe dominate dissociation channel for HXeCl,while two dissociation channels are possible and com-petitive for HXeF.Based on the obtained potentials,vibrational energy levels of HXeCl and HXeF arecalculated using the Lanczos algorithm.Our theoretical results are in good agreement with the avail-able observed values.Particularly,the calculated fundamental frequency of the H—Xe stretching vi-bration including the Xe matrix effect of HXeCl is found to be 1666.6 cm-1,which is only 17.6 cm-1higher than the recently observed value of 1649 cm-1.     

Structure and potential energy surface of K+·CX2

The potential energy surface of K⁺·CO₂ and K⁺·CS₂ complexes are determined at high levels of ab initio theory (CCSD), B3LYP, and MP2 using different basis sets. These calculations predict the existence of three minima for K⁺·CO₂ complex; linear, quadrilateral and Y-shape, and two minima for K⁺·CS₂ complex; bent and Y-shape. In addition, the binding energies, and other thermodynamic quantities are calculated at different levels of theories. Our calculations show that the difference in the global minima geometries of both complexes is mainly due to the sign of the quadrupole moment of CO₂ and CS₂ molecules.     

Connection between the Ogilvie and the Murrell–Sorbie potential energy functions

Formulas for relating the parameters of the Murrell–Sorbie and the Ogilvie potentials are developed herein. Unlike the Simons–Parr–Finlan function, the Ogilvie potential is more easily connected with the Murrell–Sorbie potential, as evident from the longer range of agreement. The relationship is useful for generating high order Ogilvie potentials that exhibit the dissociation energy without experimentation, and for using the Murrell–Sorbie parameters in molecular softwares that adopt the Ogilvie function in their algorithms. The relations are invertible so that Ogilvie parameters from spectroscopic data can be applied in molecular softwares that employ Murrell–Sorbie potentials.      

An SCF-CI method for determining the potential energy surface of a triatomic molecule

A self-consistent-field (SCF)-configuration interaction (CI) (SCF-CI) method for determining the potential energy surface of a triatomic molecule from the observed vibrational band origins has been suggested. By this method, the SCF-CI procedure in the internal coordinates is used to calculate the vibrational bond origins and their first derivatives with respect to parameters in the potential energy function using the exact vibrational Hamiltonian, and the optimizer LMF in the nonlinear-squares problem is employed to optimize parameters in the potential energy function. This approach is used to optimize the potential energy function of the water molecule. The standard deviation of this fitting to the 70 observed band origins is 1.154cm-1.     

Positron–electron correlation-polarization potential model for positron binding in polyatomic molecules

Positron binding energies (PBEs) of 41 polyatomic molecules were calculated using the positron–electron correlation-polarization potential (CPP) approach and compared with experimentally measured values. In this approach, the short-range positron–electron potential is modeled using the density-functional expression, whereas the long-range potential is approximated by the attractive polarization potential. The positron–electron CPP model based on local-density approximation yields larger PBEs than experimental values; however, the calculated values can be substantially improved by introducing generalized gradient approximation. We also investigated the conformational dependence of PBEs for representative molecules.     

Quantum-chemical study of the singlet potential energy surface of the nitrene—dioxygen system

The potential energy surfaces of the HN—O₂ and PhN—O₂ systems were calculated by the MP2 and B3LYP methods. The mechanism of photooxidation of azides was refined. Photooxidation produces the nitrene—O₂ adducts with dioxaziridine and non-cyclic structures. The parameters of IR spectra of the adducts were calculated. The rearrangement of dioxaziridine to a nitro compound is likely a reason for chemiluminescence accompanying the photooxidation of azides.     

Two-configuration potential energy surface for the Ca + HF → CaF + H reaction

The three-dimensional potential energy surface for the reaction of Ca and HF has been obtained from a two-configuration direct minimization method using an extended GTO basis set. Features of the surface were examined by fitting calculated values using cubic splines. The height of the barrier to reaction was found to be quite insensitive to the direction of the impinging Ca atom for a wide range of angles of approach. The transition state occurs in the exit channel at an angle of approach of 77°, 33.6 kcal/mole (or 14.8 kcal/mole if the correlation energy error is considered) above the reactants asymptote. A further analysis of the basis set superposition error and the correlation energy error is presented, these results modify the dynamical properties of the system and give a criterion to test the validity of the previously reported two-configuration PES for BeFH and MgFH systems.     

Differential diffusion quantum Monte Carlo method: determination of potential energy surfaces of molecules

A differential approach for self-optimizing diffusion Monte Carlo calculation was proposed in this paper, which is a new algorithm combining three techniques such as optimizing, diffusion and correlation sampling. This method can be used to directly compute the energy differential between two systems in the diffusion process, making the statistical error of calculation be reduced to order of 10-5 hartree, and recover about more than 80% of the correlation energy. We employed this approach to set up a potential energy surface of a molecule, used a "rigid move" model, and utilized Jacobi transformation to make energy calculation for two configurations of a molecule having good positive correlation. So, an accurate energy differential could be obtained, and the potential energy surface with good quality can be depicted. In calculation, a technique called "post-equilibrium remaining sample was set up firstly, which can save about 50% of computation expense. This novel algorithm was used to study the potenti     

Evaluating the biochemical potential for Irbid’s food waste as a source for energy

Solid waste generated from the city of Irbid is all directed to a local unlined landfill with no gas collection system. More than half of the waste stream is made of food waste which turns the landfill to a concentrated point source of greenhouse gases. To assess the impact that the landfill has at the moment on global warming and to evaluate the future value of the city’s waste as a source of biogas, 20 trucks of municipal solid waste were sampled. In addition, food waste samples from Irbid were collected from five different sources for 12 weeks to quantify their generation rates and to measure their components and chemical composition. Irbid’s solid waste was found to have a distinctively high biodegradable content 68%, which is typical of developing countries’ solid waste. Physical assessment showed that 38%, 6%, 33% and 23% of the food waste was rice, meat, fruits and vegetables, and bread, respectively. Measured methane yields for the volatile solids (VS) in rice, meat, fruits and vegetables, and bread were 362, 499, 352 and 375 mL/g VS, respectively. A representative food waste sample was created to test the actual methane yield and compare it to calculated one. Actual methane yield (414 mL/g VS) was greater than the calculated value (370 mL/g VS) based on food type proportions and their specific methane yield. To assess the anaerobic biodegradability of food waste, a stoichiometric calculation of the methane production was made based on the elemental analysis of all food waste elements. Food waste was found to have the chemical formula C₇H₁₆O₅N and calculated methane potential of 447 mL/g VS; indicating 85% anaerobic biodegradability of food waste, which makes Irbid’s waste perceived as a valuable source of energy.     

Interpolated potential energy surfaces: How accurate do the second derivatives have to be?

A global potential energy surface for the water dimer is constructed using the modified Shepard interpolation scheme of Collins et al. According to this interpolation scheme, the energy at an arbitrary geometry is expressed as a weighted sum of Taylor series expansions from neighboring data points, where the energy and derivative data required are obtained from ab initio calculations. For some ab initio methods, errors are introduced into the second derivative matrix, either by numerical differencing of ab initio energies or numerical integration during the ab initio calculation. Therefore, we test the accuracy required of the second derivative data by truncation of the exact second derivatives to a series of approximate second derivatives, and assess the effect on the results of a quantum diffusion Monte Carlo (QDMC) simulation. Our results show that the calculated zero-point energy and wave function histograms converge to within the numerical uncertainty of the QDMC simulation by inclusion of either three significant figures or three decimal places in the second derivatives.     

A procedure of determining potential energy surfaces of triatomic molecules from inversion of spectroscopic data

In this paper,we have suggested an iterative procedure of optimization of the linearparameters in an analytic potential energy function for a triatomic molecule,by combining both variational and second order perturbation methods.The most important feature of this procedure is that the objective function is an analytical expression which can be optimized easily.The application to the water molecule is presented.     

O?+?C2H4 potential energy surface: lowest-lying singlet at the multireference level

Extensive density functional theory (DFT) calculations have been performed to develop a force field for the classical molecular dynamics (MD) simulations of various azobenzene derivatives. Besides azobenzene, we focused on a thiolated azobenzene’s molecular rod (4′-{[(1,1′-biphenyl)-4-yl]diazenyl}-(1,1′-biphenyl)-4-thiol) that has been previously demonstrated to photoisomerize from trans to cis with high yields on surfaces. The developed force field is an extension of OPLS All Atoms, and key bonding parameters are parameterized to reproduce the potential energy profiles calculated by DFT. For each of the parameterized molecule, we propose three sets of parameters: one best suited for the trans configuration, one for the cis configuration, and finally, a set able to describe both at a satisfactory degree. The quality of the derived parameters is evaluated by comparing with structural and vibrational experimental data. The developed force field opens the way to the classical MD simulations of self-assembled monolayers (SAMs) of azobenzene’s molecular rods, as well as to the quantum mechanics/molecular mechanics study of photoisomerization in SAMs.     

An ab initio quasi-diabatic potential energy matrix for OH(2Σ) + H2

A diabatic potential energy matrix for three electronic states of OH(3) has been constructed by interpolation of multi-reference configuration interaction electronic structure data. The reactive, exchange and non-reactive quenching dynamics are investigated using surface hopping classical trajectories. Classical trajectory simulations show good agreement with cross molecular beam data for the OH((2)Σ) + D(2) → HOD + D reaction.     

Proton-CO2(X1Σ) orientational anisotropy from an ab initio SCF potential energy surface

Ab initio calculations have been carried out, over an extended range of relative distances and geometries, to obtain the SCF potential energy surface that describes the ground-state interaction between H⁺ and the CO₂ molecular target treated as a rigid rotor. Various aspects of the forces are discussed with special attention to their general dependence on internal angle, affecting rotational excitation of CO₂ during colisional encounters. The corresponding multipolar coefficients are calculated and the asymptotic behaviour of their lowest terms compared with results from perturbation theory, thus providing some qualitative indication on the expected energy-transfer probability in molecular-beam experiments or in ion diffusion processes.     

A generalized harmonic oscillator model of energy level particles and NNS distribution——A unified theory for NNS distribution of energy level fluctuation spectra

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