If I really understand the UFF authors eventually abandoned the use of partial charges and furthermore do not recommend them.

There is an article in which the authors have used UFF as a generic force field and afterward they have implemented Atomic Partial Charges via Hartree-Fock calculations at 6-31G* basis set. You know they have used CHELPG method for fitting Partial charges to the atomic centres. The question is that as I do the same process with Dmol3 package I achieve Mullikan Charges which are completely different of what they have got. Where is the problem? how can I convert Mullikan Charges to CHELPG Partial charges?

The information about electron (charges) probability (distribution) is carried by wave functions after successful convergence of SCF. Then every analysis for calculation of the charges starts from these wave functions and develops a set of charge (distribution) based on the underlying theory. So, you cannot convert Mullkin charges to any other type. you need to required analysis on the wavefuntions you got from you HF calculations. In whatever software you use, there is an input parameters that controls the available analysis. I am not sure about the Dmol3, but Gaussian supports, almost, all possible options.

Accurate procedure for the determination of ESP charges was implemented in the Firefly program. It's points are placed on spherical surfaces. However, I would recommend a large points density. This applies to all programs. For some reason it taking too few points, although this issue thoroughly versed in literature.

How many heavy atoms in your molecule?

Will be the model used in the aquatic environment?

The only heavy atom that I have here is copper. All other atoms are carbon, oxygen and hydrogen. the model is going to be used in solid and gas environment only.

Sorry, I'm not exactly said. Heavy in this case mean "not hydrogen".

Unfortunately I am not specialized on solid state, here things might be different.

Therefore I tell you how I would have acted. I hope specialists will correct me.

I would take Ascalaph graphic program, and install Firefly it in.

If the molecule has a lot of free torsion angles, I would use the interface parameters from Ascalaph to Firefly by default. If the molecule is conformationally rigid then VDWSCL parameter would increase from 1.1 to 1.5 angstroms (so better reproduced dipole and quadrupole moments).

If the size of the molecule allows fit tasks in memory, then I would use pbe0 (or b3lyp) with def2-tzvppd basis set, if no then def2-svpd.

You can determine the partial charge in each atom in a molecule using NBO(Natural bond orbital) test -Gaussian 09 program

@Jamal Dawoud

Since we are talking about ESP charges, then apparently meant the description of the electrostatic potential around the molecules for molecular mechanical calculations. Neither natural nor Mullikеn charges are not suitable for this purpose. They are designed for other purposes and poorly describe the electrostatic properties. The easiest way to get the appropriate charges is PDC. Either it should be done as done in OPLS or CHARMM, but it is much harder.

Dear Ahmad

You can easily find the accurate atomic charges using pop=esp at the command line in Gaussian software package. By the way, pop=chelpg can also be used.

http://www.sciencedirect.com/science/article/pii/S0921510704005926 

Net atomic charges, also called partial charges or partial atomic charges, are commonly used for two different purposes: (1) to quantify the transfer of electrons between atoms in a material as computed by quantum chemistry and (2) to construct force-fields (for classical molecular dynamics or Monte Carlo simulations) that approximately reproduce the electrostatic potential surrounding the material. While quantum chemical topology (QCT), also called quantum theory of atoms in molecules (QTAIM), often describes electron transfer in buried atoms with reasonable accuracy, the QCT atomic charges are not suitable to construct point-charge models for classical force-fields, because the QCT atomic multipoles are large in magnitude. (The QCT method can be used to construct accurate force-fields using multi-centered polyatomic multipole expansions, but these are more complicated than the simple point-charge based force-fields often used.) On the other hand, electrostatic potential derived point charges (e.g., CHELP, CHELPG, ESP, Merz-Singh-Kollman, REPEAT) have reasonable accuracy for reproducing the electrostatic potential surrounding the material, but these do not accurately describe electron transfer for buried atoms.

The density –derived electrostatic and chemical (DDEC) methods solve this problem by partitioning the electron density to simultaneously reproduce atomic chemical states and the electrostatic potential surrounding the material with excellent accuracy. The latest generation (called DDEC6) is described in these papers: (a) T. A. Manz and N. Gabaldon Limas, “Introducing DDEC6 atomic population analysis: part 1. Charge partitioning theory and methodology,” RSC Advances, 6 (2016) 47771-47801 (http://dx.doi.org/ 10.1039/C6RA04656H) and (b) N. Gabaldon Limas and T. A. Manz, “Introducing DDEC6 atomic population analysis: part 2. Computed results for a wide range of periodic and nonperiodic materials,” RSC Advances, 6 (2016) 45727-45747 (http://dx.doi.org/ 10.1039/C6RA05507A). It gives net atomic charges that are closely correlated to many experimental properties as described in those articles. The DDEC6 method is implemented in the free Chargemol program (http://ddec.sourceforge.net) which can analyze results of GAUSSIAN, VASP, and other quantum chemistry programs.