The Torsion Scan option calculates how molecular energy changes as a selected torsion angle is rotated. Conformations can be evaluated directly or optimized while keeping the scanned torsion fixed.
To perform a torsion scan:
- Prepare a 3D ICM object and check that its atomic formal charges are correct.
- Select the four atoms defining the torsion using the pick atom button, in order. The bond between the second and third atoms is the rotation axis.
- Select MolMechanics/Quantum Mechanics/Torsion scan.
- Specify the angular interval and scan range. For example, scanning from 0° to 360° at 15° intervals generates 25 conformations, including both endpoints. The 0° and 360° conformations represent the same torsion orientation.
- Choose the number of cores you want to use
Energy Evaluation
Psi4 calculates the energy of each saved conformation using B3LYP/6-31G* by default, without further changing its geometry. Any relaxation during the initial generation of conformations depends on the torsion scan settings.
Minimization with a Fixed Torsion
Each conformation undergoes quantum geometry optimization while the scanned torsion remains fixed at its assigned angle. The remaining geometry is allowed to relax.
This option is slower than energy evaluation alone but can provide a more representative energy profile by allowing the molecule to adjust at each torsion angle.
Results
The conformations are stored in a conformational stack, with energies reported in kcal/mol relative to the lowest-energy conformation.
Command-line references: show energy quantum and minimize quantum (with fixed torsions).