If the molecule has an A1 ground state in C2v then the lowest B1 transition would also correspond to the lowest B1 state.
In general though The ROOTS_PER_IRREP options should be set using the excited state symmetry, if you know the symmetry of the transition you can work out the final state symmetry using the product table for the group and the ground state symmetry.
Two examples from the C2v PG:
- CH2-triplet first B1 transition:
- Ground state: B1
- Excited state = B1 x B1 = A1
- ROOTS_PER_IRREP = [ 1, 0, 0, 0]
- H20-singlet first B1 transition:
- Ground state: A1
- Excited State = B1 x A1 = B1
- ROOTS_PER_IRREP = [0, 0, 1, 0]
What does that mean? Should the array not be [0,0,1,0]?
If the ground state is A1, then yes that would be the correct way to do the calculation you want. However if the ground state is not totally symmetric and you want the first B1 transition and not the first B1 state you will have to work out the symmetry of the final state to set the ROOTS_PER_IRREP option.