ABSTRACT:The structural and electronic characteristics of the M 4 R 4 with M ϭ C, Si, Ge and R ϭ H, F, Cl, OH, MgH, BH 2 , NH 2 Li are studied by ab initio calculations based on density functional theory using the hybrid B3LYP functional and 6-311ϩϩG* basis set. The results of natural bond orbital analysis (NBO), the analysis of structural parameters (bond lengths and bond angle, dihedral angle), atoms in molecules based theory (AIM) topological parameters such as the characteristics of bond critical points (electron densities and their Laplacians) leads to the conclusion that interamolecular interaction due with OH, MgH 2 , and NH 2 substitutions are different from other derivatives. The cyclobutadiene ring and its heavy versions display very different geometrical structures. The cations and anions of C 4 R 4 , Si 4 R 4 , and Ge 4 R 4 were studied to determine the effect of substitution on structure and stability. It has been shown that substitutions with ability of forming hydrogen bonds, bound to the silicon and germanium atoms are able to change the properties of the four-member rings more than the other substitutions. Comprehensive analyses of calculated NICS (nucleusindependent chemical shifts), polarizability, electrophilicity, and electron density are done.

Molecular dynamics simulations were performed to study thermodynamics and structural properties of expanded caesium fluid. Internal pressure, radial distribution functions (RDFs), coordination numbers and diffusion coefficients have been calculated at temperature range 700-1600 K and pressure range 100-800 bar. We used the internal pressure to predict the metal--non-metal transition occurrence region. RDFs were calculated at wide ranges of temperature and pressure. The coordination numbers decrease and positions of the first peak of RDFs slightly increase as the temperature increases and pressure decreases. The calculated self-diffusion coefficients at various temperatures and pressures show no distinct boundary between Cs metallic fluid and its expanded fluid where it continuously increases with temperature. 682YEGANEGI, MOEINI and DOROODI performed in 1952 11 and the first momentous molecular dynamics (MD) simulation for Cs was performed in 2001. 12 The diffusion and shear-viscosity coefficients and velocity auto correlation function have been reported for Na, K, Rb and Cs near the melting point. [13][14][15] The simple but useful expressions for the relationship between structural and surface properties of liquid metals have been presented by employing the hard-sphere description near the melting point. 16,17 Also, MD simulations have been performed to calculate the structural and transport properties of liquid alkali metals by using reformed Morse potential near the melting point. 18 The embedded atom method potentials have been calculated for the alkali metals along the melting line of the metal and the discrepancy between the simulated energy, and the actual energy of the metal at high temperatures has been discussed. 19 Recently, thermodynamic quantities of the alkali metals 20 and their cluster size distributions 21 have been studied based on the static crystalline properties and using ab initio potential respectively.Gupta proposed a semi-empirical potential to study the metallic fluids by molecular dynamics. 22 The Gupta potential function has been applied successfully by many researchers to describe the structure and energy of Pb 23 , Ni, Ag, Au 24 and Cu-Au, Al, Zn and Cd clusters. 25 Also, Gupta potential has been used to study the melting of Na and Cs clusters in order to investigate the effects of the geometric and electronic magic numbers on the melting temperature as a function of cluster size. [26][27][28] The electronic structure of alkali metal fluids strongly depends on the thermodynamic state of the system. When Cs fluid expands, two transitions can occur, liquid-vapour and metal-non-metal one. 9 In fact, a high-density liquid metal is a conductor, while it becomes a non-metal insulator when turned into low-density vapour. Near the critical point the transition from a liquid metal to a non-metal takes place. 29 This transition implies that the nature of the particle interactions must change dramatically, from metallic to a van der Waals-type interaction. 30 The particle interactions in dense...