Level Shift / Direct SCF

Level Shift for Difficult Convergence

task: UHF STO-3G

MOLECULE
  Multiplicity  2
END

GEOMETRY
  Li  0.0  0.0  0.0
END

SCF
  Shift    0.5
  MaxCycle 150
END

Direct SCF — Large Molecule

task: RHF 6-31G*

GEOMETRY
  C     -1.796395   -0.568749   -0.005321
  C     -0.585071    0.125222   -0.004539
  C      0.625618   -0.569857   -0.004391
  C      1.831897    0.128238   -0.003610
  C      1.832534    1.521736   -0.002974
  C      0.626894    2.220934   -0.003119
  C     -0.584430    1.526963   -0.003900
  C     -1.795119    2.222042   -0.004049
  C     -3.001397    1.523947   -0.004830
  C     -3.002034    0.130449   -0.005466
  H     -1.806316   -1.656277   -0.005820
  H      0.634545   -1.657393   -0.004883
  H      2.773551   -0.415222   -0.003495
  H      2.774685    2.064334   -0.002365
  H      0.636815    3.308462   -0.002619
  H     -1.804045    3.309579   -0.003557
  H     -3.943052    2.067407   -0.004944
  H     -3.944185   -0.412149   -0.006075
END

SCF
  Direct   true
  MaxCycle 100
END

Notes

• Level Shift (Shift): Adds a constant to virtual orbital energies, helping convergence for systems with small HOMO-LUMO gaps or near-degeneracies.
• Direct SCF (Direct true): Recomputes ERIs each SCF iteration instead of storing them. Trades memory for CPU time. Essential for large systems.
• Combined: Use both for difficult large systems:

SCF
  Direct   true
  Shift    0.3
  MaxCycle 200
END

• Direct SCF uses symmetry-accelerated cache lookup and Schwarz screening for efficiency.