I am currently working with the NEO (nuclear–electronic orbital) methods in Q-Chem, and I would greatly appreciate your guidance on how spin is defined and controlled in the nuclear and electronic components of a NEO calculation.
I have a few specific questions:
Total Spin Definition
How is the total spin of a NEO system defined?
Is it the sum of an electronic spin contribution and a separate nuclear spin contribution?
For a proton (spin ½), does the nuclear spin explicitly enter the total spin quantum number reported by Q-Chem?
Orientation of Nuclear Spin
For a proton or triton (spin = ½), does NEO-Q-Chem treat the nuclear spin as “up” or “down,” or is the nuclear spin not explicitly oriented at all?
Determining Final Spin State
In the output of a converged NEO calculation, how can we determine the final spin state of the coupled nuclear–electronic system?
Are the electronic and nuclear spin contributions reported separately?
Spin Constraints
Is there a way to fix the nuclear spin or impose a particular nuclear spin alignment?
Are user-defined nuclear spin configurations (e.g., up-up, down-down) supported, or is the nuclear spin always treated in a spin-averaged way?
Below is the NEO-HF input file for Tritium trimer I constructed:
$molecule
0 4
H 0.394031000000 0.394031000000 0.000000000000
H -3.788624000000 0.394594000000 0.000000000000
H 0.394594000000 -3.788624000000 0.000000000000
$end
$rem
jobtype = opt
METHOD = B3LYP
BASIS = def2-qzvp
NEO = True
NEO_EPC = epc172
NEO_ISOTOPE = 3
NEO_N_SCF_CONVERGENCE = 8
SCF_CONVERGENCE = 8
NEO_E_CONV = 8
SCF_ALGORITHM GDM
$end
$neo_basis
H 1
S 1 1.000000
5.973 1.0
S 1 1.000000
10.645 1.0
S 1 1.000000
17.943 1.0
S 1 1.000000
28.950 1.0
P 1 1.000000
7.604 1.0
P 1 1.000000
14.701 1.0
P 1 1.000000
23.308 1.0
D 1 1.000000
9.011 1.0
D 1 1.000000
19.787 1.0
F 1 1.000000
10.914 1.0
F 1 1.000000
20.985 1.0
H 2
S 1 1.000000
5.973 1.0
S 1 1.000000
10.645 1.0
S 1 1.000000
17.943 1.0
S 1 1.000000
28.950 1.0
P 1 1.000000
7.604 1.0
P 1 1.000000
14.701 1.0
P 1 1.000000
23.308 1.0
D 1 1.000000
9.011 1.0
D 1 1.000000
19.787 1.0
F 1 1.000000
10.914 1.0
F 1 1.000000
20.985 1.0
H 3
S 1 1.000000
5.973 1.0
S 1 1.000000
10.645 1.0
S 1 1.000000
17.943 1.0
S 1 1.000000
28.950 1.0
P 1 1.000000
7.604 1.0
P 1 1.000000
14.701 1.0
P 1 1.000000
23.308 1.0
D 1 1.000000
9.011 1.0
D 1 1.000000
19.787 1.0
F 1 1.000000
10.914 1.0
F 1 1.000000
20.985 1.0
$end
I would be grateful for any insight, or if there is a recommended reference that explains how nuclear and electronic spin are handled within the NEO framework.