Please, let me know which an open free molecular modelling program will apply the B3LYP/G-31G(d,p) density functional level of theory? Thanks. Helen
There are several free packages, see here.
Like orthocresol mentioned, you might want to be tutored by someone of experience at first. Quantum calculations are very easy to start (too easy, some might say), but harder to actually interpret correctly. The actual method can also greatly influence the results and needs to be chosen carefully (for example, B3LYP has fallen out of favor after some weird behaviours have been observed (Why the Standard B3LYP/6-31G* Model Chemistry Should Not Be Used in DFT Calculations of Molecular Thermochemistry: Understanding and Correcting the Problem); consider M06 or M062X as good alternatives)