Elijah Johnson

X‐ray diffraction study and models of liquid methane at 92 K

New x‐ray diffraction data for liquid methane near the triple point are presented. The data are analyzed using molecular scattering factors for methane which permit extraction of the carbon–carbon structure and correlation functions. Several site–site pair potentials are used with the reference interaction site model (RISM) to predict the structure and correlation functions for a dense methane‐like fluid, and these are compared to the structure and correlation functions derived from the diffraction study. In addition, the accuracy of RISM is assessed by comparison with published computer simulation calculations on methane. Our results indicate that although the RISM distribution functions for ’’tetrahedral’’ molecules such as methane hae some physically unrealistic features, they compare well with molecular dynamics results. Site–site interaction potentials give an adequate description of liquid methane, provided that attractive forces are included in the potential. Although methane molecules are ’’seen’’...

Application of the RISM theory to Lennard‐Jones interaction site molecular fluids

It seems that reference interaction site model (RISM) theory atom–atom distribution functions have been obtained directly from the RISM equations only for fused hard sphere molecular fluids. RISM distribution functions for Lennard‐Jones interaction site fluids are presented. Results presented suggest that these distribution functions are as accurate as RISM distribution functions for fused hard sphere molecular fluids.

Atom pair potentials of liquid nitrogen from diffraction data

The structure function of liquid nitrogen at 65 K and a molecular density of ρ=0.01851 A−3 has been derived from new x‐ray diffraction data. The RISM integral equation method was used to obtain a repulsive reference pair potential from this structure function. The results indicate that RISM integral equation method gives reliable repulsive reference potentials.

A theory for site–site pair distribution functions of molecular fluids

A theory for site–site pair distribution functions of molecular fluids is presented. This theory is analogous to the RISM theory, but it is not restricted to interaction site pair potentials. It was derived directly from the Ornstein–Zernike equation using Fourier–Wigner series. Results of the theory for homonuclear diatomic pair potentials are compared with corresponding results obtained from the RISM theory and from computer simulation studies. The relationship between the RISM equation for homonuclear hard sphere diatomic pair potentials and the theory presented is given.

Neutron multiple scattering and absorption factors

Absorption and multiple scattering factors for neutrons are determined using the Monte Carlo integration method. This procedure gives results that are as accurate as that from other methods. It is also simpler and apparently converges more rapidly than other methods.

The neutron flux of a252Cf neutron activation analysis device using the MCNP code

This report presents results from the application of the Monte Carlo N-Particle (MCNP) computer code to the252Cf neutron activation analysis (NAA) Device in the Technical Physics Institute of the Heilongjiang Science Academy of the Peoples Republic of China. The thermal and epithermal neutron flux at the sample positions and the neutron and photon fluxes on the surfaces of the device were calculated. A comparison between the calculated and experimental thermal and epithermal neutron fluxes at sample positions yield relative errors of less than 10% for the thermal neutron flux.

The three‐particle distribution function of lattice gases

A variational method for determining the three‐particle distribution function of classical systems from the single‐particle and pair distribution functions is used to treat one‐dimensional nearest‐neighbor interaction lattice gases. Apparently, no approximations were used to develop this method. Since the three‐particle distribution function of finite range interaction lattice gases can be determined by using the transfer matrix method, the suitability of the variational method for determining three‐particle distribution functions is examined. The results show that the variational method is successful.

A theoretical method for determining particle distribution functions of classical systems

An equation which involves the triplet distribution function and the three‐particle direct correlation function is obtained. This equation was derived using an analogue of the Ornstein–Zernike equation. The new equation is used to develop a variational method for obtaining the triplet distribution function of uniform one‐component atomic fluids from the pair distribution function. The variational method may be used with the first and second equations in the YBG hierarchy to obtain pair and triplet distribution functions. It should be easy to generalize the results to the n‐particle distribution function.

A perturbation method for classical atomic fluids which have a repulsive pair potential

A new method for choosing a reference pair potential for atomic fluids is presented. This method and the optimized cluster theory are used to calculate pair distribution functions for a number of classical fluids. The results indicate that pair distribution functions obtained by this method are very accurate for fluids which have a repulsive pair potential.

Particle distribution functions of lattice gases

An apparently exact variational method for obtaining the three‐particle distribution function of classical, pairwise additive potential energy function systems from the pair distribution function is used to treat two‐ and three‐dimensional lattice gases. The results obtained strongly support the view that the variational method is exact. This method is also used in conjunction with the integrated Buff–Brout equation to obtain the pair and the three‐particle distribution functions of two‐dimensional lattice gases from the interparticle interactions.