We derive an equation for calculation of molecular diameter of dense fluids, with using simultaneous Lennard-Jones (12-6) potential function and the internal pressure results. Considering the internal pressure by modeling the average configurational potential energy and then taking its derivative with respect to volume to a minimum point of potential energy has been shown that molecular diameter is function of the resultant of the forces of attraction and the forces of repulsion between the molecules in a fluid. The regularity is tested with experimental data for 10 fluids including Ar, N 2 , CO, CO 2 , CH 4 , C 2 H 6 , C 3 H 8 , C 4 H 10 , C 6 H 6 , and C 6 H 5 CH 3 . These problems have led us to try to establish a function for the accurate calculation of the molecular diameter based on the internal pressure theory for different fluids. The relationship appears to hold both compressed liquids and dense supercritical fluids.

In this research, bendamustine anticancer drug in main and excited states in gaseous phase and in water and ethanol and methanol solvent phases have been studied, using DFT at MPW1PW91/6-311++G(d, p) theoretical level. The molecules of solvent affected the equilibrium structure of species. The effects of solvents have been studied by polarized continuum model (PCM). The optimized structural parameters in gaseous and in water, ethanol and methanol solvent phases as frequency data, relative energy, dipole moment, the energy of highest occupied molecular orbital (HOMO), the energy of lowest unoccupied molecular orbital (LUMO), density functional graphs and gap energies have been calculated. The results of the theoretical computations confirm that the structure of drug, in both the main and the excited states, is more stable in liquid phase than in gaseous phase, and most stable in water. The computation results confirm that the drug stability increase by increasing in polarizability of the solvent.

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