scf

This option defines how to perform an SCF calculation

charge

Value	An integer
Default	`0`

Define the total charge of the system.

spin2p1

Value	An integer
Default	`1` for even number of electrons
	`2` for odd number of electrons

Define the spin multiplicity of the system, i.e., \(2S+1\), where \(S\) is the spin of the system. For example, to consider the singlet and triplet state, one should set spin2p1 to 1 and 3, respectively.

Note that spin2p1 can be either positive or negative. Both positive and negative spin2p1 represent the same spin multiplicity, but for a SCF wave function, alpha and beta orbitals will be occupied first, respectively. For example, for 11 electrons with spin2p1 being 1, there will be 6 alpha electrons and 5 beta electrons, like most quantum chemistry software does; but when spin2p1 is -1, there will be 5 alpha electrons and 6 beta electrons.

For odd number of electrons, a positive spin2p1 will first occupy alpha orbitals. If you want the beta orbitals to be occupied first, use a negative spin2p1. For example:

scf
  charge  0
  spin2p1 3
end
mol
  H 0. 0. 0.
  H 0. 0. 1.
end

In this case, the molecule will have 2 alpha electrons and 0 beta ones. For another input:

scf
  charge  0
  spin2p1 -3
end
mol
  H 0. 0. 0.
  H 0. 0. 1.
end

The molecule will have 2 beta electrons and 0 alpha ones.

type

Value	`R` for restricted SCF (alpha and beta orbitals are restricted to be identical)
	`U` for unrestricted SCF (alpha and beta orbitals are not necessarily identical)
Default	`R` for singlet state
	`U` for other states

For singlet states, both restricted and unrestricted SCF are available. Unrestricted SCF is very useful in treating spin polarized systems. For non-singlet states, only the unrestricted one can be used.

max_it

Value	A non-negative integer
Default	`128`

Define the maximum number of SCF iteration.

Hint

You can set max_it to 0 to do a non-iterative calculation.

energy_cov

Value	A real number
Default	`1.E-6`

The energy convergence threshold for SCF calculations.

density_cov

Value	A real number
Default	`1.E-8`

The density matrix convergence threshold for SCF calculations.

Hint

The SCF calculation is determined to be convergent when both the energy and density convergence conditions are satisfied.

nodiis: Do not use direct inversion of the iterative subspace (DIIS) convergence acceleration algorithm.

print_MO: Print molecular orbital coefficients. Without this, only molecular orbital energies and occupancies are printed.

schwarz

Value	A real number
Default	`1.E-10`

Define the Schwarz screening tolerance. All two-electron integral contributions below this tolerance will be discarded to speed up calculations. A positive real number is needed.

Warning

Do not set a too large value (like 1.E-5). It may leads to wrong results.

do_tso

Instead of doing ordinary SCF, do target state optimization (TSO) SCF. This is a powerful method for calculating excited and diabatic states. For theoretical details, please refer to:

J. Chem. Theory Comput. 2023, 19, 6, 1777-1789

Tutorials of using TSO-DFT can be found here:

TSO-DFT for Excited State Energies