Monte Carlo simulation of carbon monoxide,carbon dioxide and methane adsorption on activated carbon

In this study, the adsorption capacity of pure and activated carbon with regard to carbon monoxide (CO), carbon dioxide (CO₂) and methane (CH₄) gases at 298?K and pressure from 0.01 up to 2.0?MPa has been investigated computationally. Computational work refers to Monte Carlo (MC) simulation of each adsorbed gas on a graphite model with varying density of activation sites. The Grand Canonical Monte Carlo (GCMC) simulation technique was employed to obtain the uptake of each adsorbed gas by considering a graphite model of parallel sheets activated by carboxyl and hydroxyl groups, as observed experimentally. The simulation adsorption data for these gases within the examined carbon pore material are presented and discussed in terms of the adsorbate fluid molecular characteristics and corresponding interactions between adsorbate species and adsorbent material. We found that the simulated adsorption uptake of the examined graphite model under these conditions with regard to the aforementioned fluids increases in the order CO?<?CH₄?<?CO₂.     

Physical adsorption of gases at high pressure

The adsorption of a (modified) Lennard-Jones gas on a plane, rigid solid surface is studied using the grand canonical ensemble Monte Carlo method for a wide range of physical conditions. The shapes of the adsorption isotherms are in good qualitative agreement with those observed experimentally. Multilayers adsorption is observed at temperatures from just below to well above the critical temperature, T_c . As the pressure is increased for T?T_c the adsorbed film tends to become thick in the sense that the outer adsorbed layer becomes highly deloealized and asymmetric.     

Spin-polarized electron–electron quantum bilayers: Finite width effect

We study the effect of finite width on the ground-state of a spin-polarized electron–electron quantum bilayers (EEBL) system at temperature T=0. Correlations among carriers are treated beyond the static mean-field theories by using the quantum or dynamical version of Singwi, Tosi, Land and Sjölander (qSTLS) theory. Numerical results are presented for the pair-correlation function, the ground-state energy, the static density susceptibility, and the static local-field correction factor as a function of density parameter r_sl and interlayer spacing d. Interestingly, we find that the inclusion of finite width lowered the critical density, for the onset of Wigner crystal (WC) ground-state, as compared to the similar recent study of spin-polarized EEBL system without finite width effect. Further, spin-polarization effect is seen to introduce a marked change in the ground-state energy of the EEBL system as compared to the results of unpolarized EEBL system with finite width. Results of ground-state energies are also compared with the recent diffusion Monte Carlo (DMC) and variational Monte Carlo (VMC) simulation studies of spin-polarized EEBL system with zero width.     

Spin-one-Ising model for (CO)1−x (N2) x mixtures: A finite size scaling study of random-field-type critical phenomena

A qualitative model for solid mixtures of diatomic molecules, where one species (called CO, to be specific) carries both a dipole moment and a quadrupole moment, while the other species (calledN ₂) has only a quadrupole moment, is studied by Monte Carlo methods. We use spinsS _i =±1 to represent the orientations of the CO electric dipole moment, if the lattice sitei is taken by a CO molecule, whileS _i =0 if the site is taken by anN ₂ molecule. Assuming nearest-neighbor antiferroelectric interactions between CO molecules, and a bilinear dipole-quadrupole coupling between CO andN ₂, the randomly quenchedN ₂ molecules act like random fields do in the random field Ising model. In previous work it was already shown that this crude model is in very good agreement with experimental data in two dimensions (adsorbed layers), where the random fields induces a rounding of the transition. Here Monte Carlo simulations of the three-dimensional version of this model are presented and analyzed with finite size scaling concepts. As expected from the theory, a behaviour qualitatively different from the two-dimensional case is detected. The Monte Carlo data provide qualitative evidence that the random field induces crossover to an universality class with critical exponents distinct from the pure Ising model, but it is not feasible to us to study large enough systems that would allow a reliable estimation of these exponents. But the results show that dilution without dipole-quadrupole coupling has much less drastic effects on the critical behavior, and that in the presence of this coupling very small impurity concentrations do indeed change the critical behavior.This paper is dedicated to Professor Herbert Wagner on the occasion of his 60th birthday     

Experimental and theoretical studies of linear polyatomic molecules on surfaces of low symmetry: C2N2- on Pt(110)

The adsorption and surface dissociation of C₂N₂ on Pt(110), and the desorption kinetics of cyanide adlayers have been studied by LEED, Auger spectroscopy, and thermal desorption. The unusual adsorption kinetics which are accompanied by a (1 × 2) → (1 × 1) change in the surface periodicity are discussed in terms of absolute rate theory. The desorption process is modelled using 2D Monte Carlo and 1D analytical methods; these calculations indicate that the adlayer is only partly equilibrated — a conclusion which is confirmed by the results of CO coadsorption experiments. Values are deduced for the desorption parameters which suggest that the CN molecule is multiply bonded to the Pt surface.     

Surface Critical Behavior of Two-Dimensional Dilute Ising Models

Ising models with nearest neighbor ferromagnetic random couplings on a square lattice with a (1, 1) surface are studied, using Monte Carlo techniques and a star-triangle transformation method. In particular, the critical exponent of the surface magnetization is found to be close to that of the perfect model, Β₁ = 1/2. The crossover from surface to bulk critical properties is discussed.     

Cl2 adsorption on MgO(001) surface supported sodium monolayers: a density functional theory study

The adsorption of Cl₂ Na monolayers supported on the MgO(001) surface has been studied by the density functional method using cluster models embedded in a large array of point charges (PCs). The value of PCs was determined by charge self-consistent technique. The results indicate that Na-promoted MgO(001) surface is an efficient catalyst toward Cl₂ adsorptive decomposition. Besides, it was found that the role of the MgO(001) surface is not passive, which is different from CO adsorption on MgO(001) surface supported Na metal monolayers. The analysis of band and projected density of states indicates that the electron transfer from the surface Mg 3s valence orbital and Na 3s valence orbital to the anti-bonding σ^∗ orbital of Cl₂ is the source of the Cl₂ bond weakening. This is also different from the CO adsorption on MgO(001) surface supported Na metal monolayers, where only the electrons from the Na valence orbital are transferred to the anti-bonding π^∗ orbital of adsorbed CO. Our study suggests that the essence of catalysis is different for CO and Cl₂ adsorption on Na metal monolayers supported an MgO(001) surface.     

Quantum Monte Carlo study of small hydrocarbon atomization energies

A benchmark study of atomization energies is reported for 22 hydrocarbons using single determinant trial functions in the diffusion Monte Carlo (DMC) variant of the quantum Monte Carlo (QMC) method. The DMC atomization energies are compared to experiment, a complete basis set approach (CBS-Q), density functional theory with the B3LYP functional, and coupled-cluster singles, doubles and perturbative triples, CCSD(T), methods. Comparison of the DMC results to experiment yields a mean absolute deviation of 1.9kcalmol^?1, which is comparable to that of the B3LYP/cc-pVQZ (1.7kcalmol^?1) level of theory, but less accurate than that of CBS-Q (1.1kcalmol^?1). DMC performs similarly for both closed-shell and open-shell molecules with mean absolute deviations of 2.1kcalmol^?1 for the former and 1.7kcalmol^?1 for the latter systems. The use of experimental zero-point energies (ZPEs), rather than scaled B3LYP ZPEs, is found to have negligible effect on DMC atomization energies. The latter reported here provide a baseline from which further improvement in the calculation of DMC atomization energies, including the use of multi-determinant and other trial function improvements, can be measured.     

Classical calculation of proton collisions with ethylene

Single electron capture and single ionization total cross sections in collisions of proton with ethylene are calculated for an energy range 25 keV ≤ E ≤ 150 keV, using the classical trajectory Monte Carlo method. Multi-center model potentials are employed to represent the interaction of the active electron on each molecular orbital with the C₂H₄⁺_{4}^{+} core. The results are compared with experimental results for single electron capture.     

Nickel carbonyl adsorption on the Ni(111) surface

Overlayers formed by the adsorption of Ni(CO)₄ in CO on the Ni(111) surface at 100 K were characterized using high resolution electron energy loss spectroscopy and thermal desorption spectroscopy. At temperatures below 135 K, molecular nickel carbonyl adsorbs on the CO saturated Ni(111) surface as suggested by several observations. Vibrational transitions characteristic of molecular Ni(CO)₄ are dominant. The energy dependence of both the elastic and inelastic electron scattering cross sections are dramatically altered by Ni(CO)₄ adsorption. All of the mass spectrometer ionization fragments typical of molecular Ni(CO)₄ are observed in the narrow thermal desorption peak at 150 K. The inelastic scattering cross sections for both adsorbed nickel carbonyl and adsorbed CO on the Ni(111) surface suggest that a nonresonant dipole scattering mechanism is dominant.     

Universal crossover function and non-universal order-parameter profiles for critical adsorption

We present a crossover theory for critical adsorption in a semi-infinite system. The predictions are compared with Monte Carlo simulations for an Ising model with a surface and with experimental measurements for the adsorption of liquid SF₆ on graphite. In both cases we find good agreement with theory.     

Effect of hole doping on the electronic structure and the Fermi surface in the Hubbard model within norm-conserving cluster pertubation theory

The concentration dependences of the band structure, spectral weight, density of states, and Fermi surface in the paramagnetic state are studied in the Hubbard model within cluster pertubation theory with 2 × 2 clusters. Representation of the Hubbard X operators makes it possible to control conservation of the spectral weight in constructing cluster perturbation theory. The calculated value of the ground-state energy is in good agreement with the results obtained using nonperturbative methods such as the quantum Monte Carlo method, exact diagonalization of a 4 × 4 cluster, and the variational Monte Carlo method. It is shown that in the case of hole doping, the states in the band gap (in-gap states) lie near the top of the lower Hubbard band for large values of U and near the bottom of the upper band for small U. The concentration dependence of the Fermi surface strongly depends on hopping to second (t′) and third (t″) neighbors. For parameter values typical of HTSC cuprates, the existence of three concentration regions with different Fermi surfaces is demonstrated. It is shown that broadening of the spectral electron density with an energy resolution typical of contemporary ARPES leads to a pattern of arcs with a length depending on the concentration. Only an order-of-magnitude decrease in the linewidth makes it possible to obtain the true Fermi surface from the spectral density. The kinks associated with strong electron correlations are detected in the dispersion relation below the Fermi level.     

On-top density in the nonlinear metallic screening and its implication on the exchange-correlation energy functional

In comparison with the accurate data on the on-top electron density n(0) in the proton-embedded electron gas with the density parameter r_s in the range 1?12 obtained by diffusion Monte Carlo (DMC) simulations, we have successfully constructed an alternative form of the exchange-correlation energy functional in the density functional theory by imposing the constraint due to the cusp theorem on the well-known Perdew–Burke–Ernzerhof (PBE) functional. Although PBE does not, our functional, referred to as the cusp-corrected PBE (ccPBE), reproduces the DMC data on n(0) in the entire range of r_s.     

Adsorption of carbon monoxide on Si x Ge4– x (x = 0–4) nano-clusters: a hybrid meta density functional study

Theoretical study of carbon monoxide adsorption on Si _x Ge_{4 ? x} (x = 0–4) nano-clusters has been carried out using advanced hybrid meta density functional method of Truhlar (MPW1B95). MG3 semi-diffuse (MG3S) and correlation consistent valence basis sets with relativistic core potential were employed to improve the results. The agreement of the calculated ionization and dissociation energies with experimental values validates the reported structures of nano-clusters and justifies the use of hybrid meta density functional method. The geometry, adsorption energy, charge distribution, and vibrational frequency of CO adsorption on all possible structures were investigated. The maximum vibrational frequency changes occur in the bridge structures while the most stable structures occur when CO adsorbs on one silicon atom in a flat surface. The changes of spin densities arising through bridged structures with higher spin multiplicities were rationalized. Adsorption energies of CO on one Si atom are by far more negative than the corresponding value for on Ge atom, at the highest being nearly ?77 and ?35 kJ mol^?1. Comparison was made of adsorbed CO bridging neighbouring and diagonal Si atoms and the former was more stable, having adsorption energy of nearly ?77 kJ mol^?1. Flat surfaces adsorb CO more favourably. Exhaustive vibrational frequency analyses were performed to confirm the local minima energy of all optimized structures.     

Modification of chemisorption properties by electronegative adatoms: H2 and CO on chlorided,sulfided, and phosphided Ni(100)

The effect of preadsorbed electronegative atoms Cl, S and P on the adsorption-desorption behavior of CO and H₂ on Ni(100) has been studied using thermal desorption, LEED and AES. It is found that the presence of the electronegative atoms causes a reduction of the adsorption rate, the adsorption bond strength and the capacity of the Ni(100) surface for CO and H₂ adsorption. The poisoning effect becomes stronger with increasing electronegativity of the preadsorbed atoms. In the case of CO adsorption the most tightly bound β₂ -state is suppressed most significantly. The variation of the initial sticking coefficient of CO as a function of the adlayer precoverage shows that at low Cl and S coverages the effective number of the influenced Ni surface atoms is more than four. The observed reduction of CO and H₂ adsorption and the difference in the poisoning effect of Cl, S and P is generally interpreted in terms of the changes in the surface electron density in the presence of electronegative atoms.     

超新星爆发环境核素56,57,59,60Co的电子俘获

基于壳模型与Random Phase Approximation理论, 利用Shell-Model Monte Carlo方法, 研究了超新星爆发环境核素^56,57,59,60Co的电子俘获与电子丰度变化率. 我们的结果与利用Aufderheide方法计算的结果进行了误差对比. 结果表明: 电子俘获率受温度和密度的影响大大增加, 甚至增加达6个数量级以上(如在ρ₇=0.43, Y_e=0.48核素^57,59,60Co). 另一方面, 随着温度和密度的增大, 电子丰度变化率大大降低, 甚至减小达5个数量级以上(如在ρ₇=5.86, Y_e=0.47核素⁵⁹Co). 通过对误差因子的分析表明, 在低温低密度环境二种结果误差较大; 而在高温高密度环境, 二种结果误差相对较小. 关键词： 电子俘获 超新星     

Alternative Oblique-Incidence Reflectometry for Measuring Tissue Optical Properties

To deduce the optical properties, the absorption coefficient SmU_aand reduced scattering coefficient μ’_s, of turbid medium, Lin et al. (Appl. Opt. 34 (1995) 2362) proposed an oblique incidence reflectometry in which the diffusion approximation was assumed. In this paper we propose an alternative method which does not assume the diffusion approximation but uses a Monte Carlo light propagation model. Two features are extracted from the diffuse reflectance distribution detected on the medium surface, and optical properties are then estimated by looking up the predetermined table generated by Monte Carlo simulations. The validity of the proposed method has been confirmed by computer simulations.     

Density-functional study of CO adsorbed on RhN （N = 2-19） clusters

We investigate the structural, electronic and adsorption properties of one single CO molecule adsorbed on Rh_N (N = 2-19) clusters, using the density-functional theory in the spin-polarized generalized gradient approximation. It is found that the structural growth model of the Rh_N clusters transforms from double layers (N = 12-16) to three layers (N = 17-19). Three different adsorption types are the atop site adsorption for N = 6, 8, 9, 11, 12, the bridge site adsorption for N = 2-5, 7, 10, 13-15, 17 and the face adsorption for N = 16, 18, 19. The adsorption abilities of Rh_N clusters are related to C-O bond length, vibrational frequency, adsorption energy and the charge transfer between CO and Rh clusters as well as the electronic density of state. With the increase of Rh cluster size, the adsorption energy of CO adsorbed on Rh_N clusters tends to be 2.2 eV-2.3 eV, which is 0.2 eV-0.3 eV larger than the theoretical value (about 2.0 eV) of CO molecule adsorption on clean Rh (111) surface.     

Electron beam induced effects on gas adsorption utilizing auger electron spectroscopy: Co and O2 on Si: I. Adsorption studies

The effect of electron beam monitored gas adsorption on the clean Si surface is studied using Auger electron spectroscopy. It is shown that the beam affects the AES adsorption signal of CO and O₂ on Si by dissociating the adsorbed molecules on the surface and subsequently promoting diffusion of atomic oxygen into the bulk. A qualitative explanation of the adsorption data is presented and the initial sticking probability of O₂ on Si (111) surface is estimated to be S0 = 0.21.     

A diffusion Monte Carlo study of small para-hydrogen clusters

An improved Monte Carlo diffusion model is used to calculate the ground state energies and chemical potentials of parahydrogen clusters of three to forty molecules, using two different p-H₂-p-H₂ interactions. The improvement is due to three-body correlations in the importance sampling, to the time step adjustment and to a better estimation of statistical errors. In contrast to path-integral Monte Carlo results, this method predicts no magic clusters other than that with thirteen molecules.