Hello,

BDE in usually calculated as the energy difference between products of dissociation and initial system. So, just detach atoms/fragments, optimize them and then subtract the energy of optimized phenol from the sum of optimized dissociation products. However, this might useful and straightforward only if one bond is broken. Keep in mind that in order to get an enthalpy values you have to calculate frequencies for each species as well. The other way to get a BDE is to calculate potential energy curve alongside specific reaction coordinate.

Hope it helps,

Nikita.

Gaussian has a very helpful white paper titled "Thermochemistry in Gaussian" that answers this question in detail, including examples. See https://gaussian.com/thermo/

it is very simple to calculate BDE by using Gaussian software. First you should perform opt+freq job for phenol, H-removed radical species (either H from ring or H from OH group) and H-atom. This calculations will provide you thermochemistry part, from there you can note down enthalpy values of all species. After that you can easily calculate BDE as suggested by Nikita S. Fedik.