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Use B3LYP-D3(BJ) for research grade calculations. We have used and recommended the world's most popular DFT method B3LYP for years, however, this functional has a serious shortcoming in not properly taking into account London dispersion forces. This is an issue for larger molecules and noncovalent complexes, but not so much for small molecules. This option is available in Gaussian by selecting "D3(BJ)" in the Empirical Dispersion dropdown menu in the WebMO Advanced tab as shown below. (D3 is the third dispersion correction introduced by Stephan Grimme some years ago; BJ stands for Becke-Johnson damping, which is a "correction on the correction.")


In ORCA, one must enter the dispersion correction manually in the input file. Also note that MP2 and CCSD(T) already correct for dispersion; do not attempt to apply D3(BJ) with these methods. Two references with further documentation are the ORCA manual and a 2025 paper on drug molecule calculations.

WebMO Demo tutorial - H₂O.

How do you interpret the thermochemistry section in the output of a Gaussian frequency job?  That is, what are ZPE, Internal Energy, Enthalpy, and how does Gaussian derive these values?  An explanation, including an example, is available here. (While rather long, it's shorter than the authoritative whitepaper on thermochemistry by Joseph Ochterski of Gaussian, Inc.)

What are Natural Bond Orbitals (NBOs)? NBOs are localized orbitals derived mathematically from the normal MOs. They reflect the molecule's bonding as suggested by a valence bond model or a Lewis formula. That is, NBOs correspond to isolated sigma (σ) bonds, pi (π) bonds, or lone pairs. For example, propene:

NBO version 7 is accessed through Gaussian or ORCA. In WebMO's  "Configure Job Options" page, select a "Natural Bond Orbitals" calculation. Both MOs and NBOs will be shown on the Results page. View the NBO 7 MANUAL here.