Dear Babu Balraj,

First of all, theoretical results are supporting evidence to the experimental observations.

HOMO-LUMO energy gap analysis are helpful to the make an effective NLO crystals for Opto-electronic devices.

From the second order hyperpolarizability, one can find whether the molecule has high degree of non-linearity or not.

Electrostatic potential, and mulliken atomic charge analysis are not relating to NLO studies. it may be the addition information about the molecule under investigation. 

i Hope this answer will help you. 

by

karthick

First of all, be sure about one thing. Depending upon the method and basis set that you are using to optimize your molecular geometry and to calculate the NLO properties, the results may vary.

When you are comparing your results with other theoretical results, their basis sets and methods are also should be taken into account.

And you mean second order hyperpolarizability (THG) or second order polarizability (SHG)? Each and every material may have an non zero second order hyperpolarizability (THG) values.

If you mean second order polarizability or first hyperpolarizability, be sure about your molecular symmetry. If your input file lacks of those details, there may arise an error.

Finally, quantum chemical calculation's ultimate objective is to understand the electronic distribution and its transitions. HOMO-LUMO, ESP analysis are calculated in this regard. From those results you may extent your interpretations. For example if a molecule has lower Eg values it may have higher polarizability.

theoretical NLO calculations  can help us to explain some Phenomenons which were found in the processes of experiments. but it is hard to indicate a compound whether having a good NLO characters