UHF in xtb

[WentongZhou]

Hi there,

I have one question about the --uhf in xtb. Most other programs use multiplicity to represent the electron spin information. But the definition here of uhf is a little different. it is number of unpaired electrons
When I tried to use xtb and xtbiff to generate interactions between CH3+ and indole. It seems that the E_ES_atom is high in positive. Is it reasonable?
xtb ch3+.xyz --chrg 1 --uhf 0 --lmo
rm xtblmoinfo 1
xtb benzene.xyz --chrg 0 --uhf 0 --lmo
rm xtblmoinfo 2
xtbiff 1 2 -sp
By using this method, I got the information below (kcal/mol):
E Pauli : 3.132
E disp ATM: 0.001
E disp 2B: -1.430
E disp total : -1.428
E ES atom: 17.383
E ES LMO: 0.223
E ES total: 17.607
E induction: 0.022
E CT: -101.289
Eint total,gas: -81.957

[cplett]

Hi,
Indeed, uhf is the number of unpaired electrons so defining --uhf 0 is correct in this case.
If the two molecules come quite close (which I would expect due to the positive charge of the CH3 fragment), the electrostatic energy can become really high due to the electron/electron repulsion and is compensated by the charge transfer term. Therefore, the result seems reasonable.