A new strategy to develop Gaussian charge models (GCMs) for molecules like ammonia, water, ethene, hydrogen sulfide, formaldehyde and benzene is presented. These molecular models comprising of positive point charges and negative Gaussian charge distributions (GCDs), which represent nuclei and continuous electron charge distribution, are found to correctly represent the ab initio Molecular Electrostatic Potential (MESP) and reproduce its essential topographical features of corresponding molecules. The models use optimized parameters: positive charges at nuclei, negative charges on GCDs, Gaussian exponent and centers. The Potential Energy Surface (PES) of water dimer has been explored using water GCMs. A good agreement has been found between PES obtained using GCMs and wave function. The Gaussian models correctly predict structure of benzene‐water complex. It is thus recommended to use GCMs for molecular dynamic simulations.

Molecular electrostatic potential and its topographical featuresFor a given molecule with N atoms located at R i with nuclear charges Z i , MESP at a point r denoted by V(r) in atomic units (a.u.) is defined equation given below where ρ(r 1 ) represents its electron density function.The rich topographical features of this three dimensional scalar field are brought out in terms of its critical points (CPs) i.e. points where first order partial derivatives of the equation vanish i.e.CPs are then characterized by nature of three eigenvalues {λi} obtained from corresponding Hessian matrix H (matrix of second order partial derivatives with elements Hij = a 2 v/x i ax j ) evaluated at the CPs.If at least one of the eigenvalues is zero then, that CP is said to be degenerate. A non-degenerate CP is found to exist when all eigenvalues are non-zero. Such a CP is represented as (R, σ) where rank (R) is the number of non-zero eigenvalues.Signature (σ) is derived from algebraic sum of the signs of eigenvalues. Rank of a non-degenerate CP of MESP scalar field is 3 and possible four types of CPs are: (3, +3), (3, -3) which represent local minimum, local maximum and (3, +1), (3, -1) are different types of saddles. These non-degenerate CPs bring out chemically significant features: a (3, +3) CP signifies presence of lone pairs or π bonds in molecules 1 -4 . Each bonded pair of atoms is reflected by presence of (3, -1) saddle. A (3, +1) CP is found to connect two minima and indicate presence of a ring or cage in the nuclear framework. The absence of non-nuclear maxima has been proven by Gadre and Pathak 5 .Supporting information / TACL 9(4) 2019 pp 453 -462

The current work demonstrates the application of topography‐based Gaussian charge models (GCMs) in studying potential energy surfaces of molecular dimers of water, ammonia, acetylene and benzene in addition to water‐ammonia, water‐acetylene, ammonia‐acetylene, water‐benzene, ammonia‐benzene and acetylene‐benzene complexes. Investigations are also carried for trimer systems of water, ammonia and their mixed compositions. The predicted geometries are in good agreement with those derived from quantum mechanical (QM) calculations and the interaction energy values forecasted deviate not more than ±2 kcal/mol from respective QM counterparts.