I found this error in the calculation of two interacting fragments, both with unpaired electrons. So, two radicals interact at a certain distance and the full system is deemed as a singlet, therefore the unpaired electron on each fragment have opposite spins. The problem came when trying to calculate the Basis Set Superposition Error (BSSE) because in the Counterpoise method you need to assign a charge and multiplicity to each fragment, however it’s not obvious how to assign opposite spins.

The core of the problem is related to the *guess *construction; normally a Counterpoise calculation would look like the following example:

#p B3LYP/6-31G(*d*,*p*) counterpoise=2
-2,1 -1,2 -1,2
C(Fragment=1) 0.00 0.00 0.00
O(Fragment=2) 1.00 1.00 1.00
...

In which the first pair of charge-multiplicity numbers correspond to the whole molecule and the following to those of each fragment in increasing order of *N* (in this case, *N* = 2). So for this hypothetical example we have two anions (but could easily be two cations) each with an unpaired electron, yielding a complex of charge = -2 and a singlet multiplicity which implies those two unpaired electrons have opposite spin. But if the *guess *(the initial trial wavefunction from which the SCF will begin) has a problem understanding this then the title error shows up:

Bad data into FinFrg
Error termination via Lnk1e ...

The solution to this problem is as simple as it may be obscure: Create a convenient guess wavefunction by placing a negative sign to the multiplicity of one of the fragments in the following example. You may then use the guess as the starting point of other calculations since it will be stored in the checkpoint file. By using this negative sign we’re not requesting a *negative multiplicity*, but a given multiplicity of *opposite spin *to the other fragment.

#p B3LYP/6-31G(*d*,*p*) guess=(only,fragment=2)
-2,1 -1,2 -1,-2
C(Fragment=1) 0.00 0.00 0.00
O(Fragment=2) 1.00 1.00 1.00
...

This way, the second fragment will have the opposite spin (but the same *multiplicity*) as the first fragment. The only keyword tells gaussian to only calculate the guess wave function and then exit the program. You may then use that guess as the starting point for other calculations such as my failed Counterpoise one.

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