Multi-level schemes

We're going to build a dipeptide, and inspect the effect of choosing the level of theory on the capping atoms.

1. Import the coordinates of a cysteine dipeptide
2. Choose the following setup:
   • XC functional: PBE (GGA)
   • Relativity: Scalar
   • Basis set: TZP
   • Frozen core: Small
   • Numerical quality: Good
3. Select the sidechain, the Cα and Hα as one region, and the others as the other region
4. Choose different setups:
   • Full QM
   • Region1(PBE) and Region2(MM-UFF)
   • Region1(PBE) and Region2(DFTB)
   • Region1(PBE) and Region2(LDA)
5. Save
6. Run

Analysis

1. Once the job has run, check the distances across the QM/QM or QM/MM boundary, and compare with the Full-QM results

Further reading

• M. Swart
  "AddRemove: A new link model for use in QM/MM studies"
  Int. J. Quant. Chem. 2003, 91, 177-183 Abstract
  10.1002/qua.10463
• M. Swart and F.M. Bickelhaupt
  "QUILD: QUantum-regions Interconnected by Local Descriptions"
  J. Comput. Chem. 2008, 29, 724-734 Abstract
  10.1002/jcc.20834