for at least one atom. As expected from our previous work, 29 these virtual sites were found almost exclusively on oxygen and nitrogen atoms. Using atom-centred point charges only, the average ESP error on these atoms was 1.87 kcal/mol, which fell to 0.53 kcal/mol after virtual site fitting. Further examples are given in the next section and a full list of molecules and ESP errors are given in the Supporting Information.