This section gives directives for Hartree-Fock and Kohn-Sham calculations. Kohn-Sham calculations are activated by invoking the keyword .DFT under **HAMILTONIAN.
Open-shell calculations correspond to either average-of-configurations (.AOC) or fractional occupation (.FOCC). The former is the default for Hartree-Fock calculations, whereas the latter is default for Kohn-Sham calculations. Note that average-of-configurations Kohn-Sham calculations are not well defined.
For each fermion irrep give the number of closed shell electrons.
The specification of the closed shell electrons is simple. For symmetry groups without inversion symmetry, there is only one fermion irrep, and you need only to specify the number of electrons.
For symmetry groups with inversion symmetry, you need to specify the distribution of the electrons in the two fermion irreps [Saue2000].
Specification of open shell(s).
For each open shell give the number of electrons and the number of active spinors.
Short example:
.OPEN SHELL
1
5/0,6
1 open shell with 5 electrons in 6 spinors (= 3 Kramers pairs) in irrep 2 (the ungerade one). Thus, the fractional occupation is 5/6.
The open shell module in DIRAC is based on average-of-configurations [Thyssen1998] . The simplest case is one electron in two spinors (= one Kramers pair). For this special case the average-of-configuration calculation gives the same result as the usual restricted open-shell Hartree-Fock. For all other cases the calculation gives the average energy of many states.
Note that the order of closed and open shells are assumed to be as in the following scheme:
Virtual orbitals Not occupied
... Open shell 2 Fractionally occupied
Open shell 1 Fractionally occupied
Closed shell
Doubly occupied; that is, the lowest molecular orbitals are doubly occupied, the next ones are occupied with the electrons of open shell no. 1, etc.
Other orderings can be achieved by using .REORDER MO’S and .OVLSEL.
To get the energies of the individual states present in the average-of-configurations, specify .RESOLVE (see also *RESOLVE).
To get the energies of (some) of the individual states present in the average of configurations, you can use the GOSCIP – COSCI module, the DIRRCI – Direct CI module, or the LUCITA.
Occupation of boson irreps in spin-free calculation. For example, for the D2h symmetry eight numbers in subsequent line, for the C2v symmetry there four occupation numbers in line.
Occupation of MJ-splitted fermion irreps (only for the linear symmetry). Next line contains sequence of occupation numbers, see the SCF output for how many.
Average-of-configuration calculation (default for open-shell Hartree-Fock).
Fractional occupation (default for open-shell Kohn-Sham) CLARIFY
.FOCC calculations are less memory-intensive than .AOC calculations. In the latter case one additional AO-Fock matrix is generated for each open shell.
.FOCC calculations are therefore an interesting option for generating start orbitals for MCSCF as well as initial convergence in open-shell Hartree-Fock.
Program is allowed to change occupation during SCF cycles. This is deactivated by default. However, the program will still try to do an automatic initial occupation if neither .CLOSED SHELL nor .OPEN SHELL is given.
An SCF-calculation (HF or DFT) may be initiated in four different ways:
The default is to start from MO coefficients if the file DFCOEF is present. Otherwise the corrected bare nucleus approach is followed. In all cases linear dependencies are removed in the zeroth iteration.
Switch off the bare nucleus correction. This correction is on per default and improves the screening of the nuclei by estimating two-electron repulsion via nuclear-attraction type integrals:
The coefficients a*j* and the exponents \(\alpha^{C_j}\) in this expression are chosen according to Slater’s rules to obtain an approximate atomic electronic density for the initial guess. For example, with one heavy element and without this correction (that is, with the bare nucleus Hamiltonian) all electrons will end up on that heavy element in the initial guess!
Start SCF-iterations from the vector file.
Start SCF-iterations from the two-electron Fock matrix from previous calculation (stored on file DFFCK2).
Start first SCF iteration with a molecular density matrix constructed from atomic densities. The keyword ATOMST is followed by input for each atomic type. The details, orbital occupation strings (see Specification of orbital strings for the syntax) and occupation, ususally correspond to those of the atomic runs, but the user may modify this at will. The syntax is explained in the parenthesis “” for each atomic type but we highly recommend to carefully check the tutorial example Atomic start guess:
.ATOMST
"SCF coefficients file name (6 characters)" "integer specifying # of occupation patterns, here: 2"
orbital occupation string #1 for type 1
occupation (real*8 value in the range of 0.0d0 - 1.0d0)
orbital occupation string #2 for type 1
occupation (real*8 value in the range of 0.0d0 - 1.0d0)
"SCF coefficients file name (6 characters)" "integer specifying # of occupation patterns, here: 1"
orbital occupation string #1 for type 2
occupation (real*8 value in the range of 0.0d0 - 1.0d0)
...
Three different criteria for convergence may be chosen:
The change in total energy is approximately the square of the largest element in the error vector or the largest change in the Fock matrix. The default is convergence on electronic gradient with 1.0D-6 as threshold. Alternatively, the iterations will stop at the maximum number of iterations.
Sometimes it may happen that the specified convergence criterion is too tight for the given basis set and/or other input parameters. In this case one needs to decide whether one should proceed with post-HF steps (like correlation calculations) or not. The program decides this by looking at a secondary convergence criterion that gives the allowed convergence. This value is by default the same as first or desired convergence criterion but can be made lower to make sure that a calculation does not abort when the convergence is slightly above threshold.
For more detailed help see SCF help on convergence troubleshooting and related pages.
Maximum number of SCF iterations.
Default:
.MAXITR
50
When restarting SCF itrations from previous molecular orbitals file (DFCMO or formatted DFPCMO), we recommend to decrease maximum number of iterations together with readjusting desired and allowed convergence thresholds. By properly set desired and allowed thresholds one can have exact number of iterations specified by .MAXITR.
Converge on error vector (electronic gradient).
2 (real) Arguments:
.EVCCNV
desired threshold allowed threshold
Threshold for convergence on total energy.
2 (real) Arguments:
.ERGCNV
desired threshold allowed threshold
Converge on largest absolute change in Fock matrix.
2 (real) Arguments:
.FCKCNV
desired threshold allowed threshold
Note that the allowed threshold may be omitted. It is then made equal to the desired threshold.
It is imperative to keep the number of SCF iterations at a minimum. This may be achieved by convergence acceleration schemes:
In DIRAC DIIS takes precedence over damping.
Change the default convergence threshold for initiation of DIIS, based on largest element of error vector.
Default:
.DIISTH
a very large number
Activate DIIS in orthogonal basis (MO) with the error vector as described above.
Activate DALTON-like DIIS using AO-basis. The error vector is
where the term \(\mathbf{f}\) is given by
Do not perform DIIS. The default is to activate .DIISMO for closed-shell calculations, and to activate .DIISAO for average-of-configurations calculations.
Do not perform damping of the Fock matrix. Damping is activated by default, but DIIS takes precedence. In case all columns in the B-matrix is removed by linear dependency, damping is activated.
Activate level shift (for virtual orbitals). Followed by a real argument (level shift).
Activate level shift (for open-shell orbitals). Followed by a real argument (level shift), one line for each open shell {Please give example}.
Change the default factor on an open-shell diagonal contribution to the Fock matrix. The default factor of one corresponds to a Koopmans interpretation of the orbital energies, however, experience shows that one in many case get better convergence without this term, so if your open-shell calculation does not converge it can often help to try again with a factor of zero on this term (or even e.g. -0.20).
Default:
.OPENFAC
1.00
The default SCF of DIRAC uses only gradient information. By adding this keyword 2nd-order optimization, using both gradient and Hessian information, is activated in case the regular SCF does not converge. This scheme is computationally more expensive and so far only available for closed-shell Hartree-Fock.
Convergence can be improved by selection of vectors based on overlap with vectors from a previous iteration. This method may also be used for convergence to some excited state.
If dynamic overlap selection is used, the vector set from the previous iteration is used as the criterion. For the first iteration either restart vectors or vectors generated by the bare nucleus approach (not*recommended) are used.*
If .NODYNSEL is given, either the restart vectors or the bare nucleus vectors are used, i.e. the overlap selection vectors are not updated in each iteration. Please note, that overlap selection based on vectors from the bare nucleus approach is not recommended.
Overlap selection is very useful together with .REORDER MO’S. This will reshuffle the vectors within the restart coefficients.
Example: First one might do a open shell calculation on Boron, this would give the P1 / 2 state. But if we restart on the P1 / 2 coefficients, interchange the p1 / 2 with the p3 / 2 orbitals, and request overlap selection, we can converge to the P3 / 2 state.
There also exists a keyword for reordering the converged SCF orbitals. This is useful for reordering the orbitals for the 4-index transformation and subsequent correlation calculations (CCSD, CI etc.) (see .POST DHF REORDER MO’S).
Activate dynamic overlap selection. The default is no overlap selection.
No dynamic update of overlap selection vectors. The default is dynamic update.
The total run time may be reduced significantly by reducing the number of integrals to be processed in each iteration:
Do not perform SCF-iterations with differential density matrix.
Default: Use differential density matrix in direct SCF.
Set \(alpha\) equal to 1.
Set thresholds for convergence before adding SL and SS/GT integrals to SCF-iterations.
2 (real) Arguments:
.CNVINT
CNVINT(1) CNVINT(2)
Set the number of iterations before adding SL and SS/GT integrals to SCF-iterations.
Default:
.ITRINT
1 1
Specify what two-electron integrals to include (see .INTFLG under **HAMILTONIAN).
Default: .INTFLG from **HAMILTONIAN.
General print level for the SCF method. For instance, value of 2 prints eignevalues during each iteration.
Default:
.PRINT
0
Controls the print-out of positive energy and negative energy eigenvalues (1 = on; 0 = off).
Default: Only the positive energy eigenvalues are printed.
.EIGPRI
1 0
In studies of electronic structure it may be of interest to eliminate or freeze certain orbitals. This option is furthermore useful for convergence, in particular to excited electronic states. A simple case is the thallium atom. The ground state 2P1 / 2 has the electronic configuration [Xe]|4f^{14}5d^{10}6s^{2}6p^1_{1/2}|. The first excited state 2P3 / 2 with the electronic configuration [Xe]4f^{14}5d^{10}6s^{2}6p^1_{3/2}| can easily be accessed by first calculating the ground state, then eliminating the 6p^1_{1/2}| from the ensuing calculation. In the final calculation the excited state is relaxed using overlap selection. The use of frozen orbitals is demonstrated in test/33.frozen: When the geometry of the water molecule is optimized with the oxygen 1s and 2s orbitals frozen, a bond angle of 96.242 degrees is found, contrary to the 90 degrees one might have expected when s-p hybridization is thus blocked [Kutzelnigg1984].
The elimination of orbitals is achieved by projecting the selected orbitals out of the transformation matrix to orthonormal basis. The selected orbitals can be expressed either in the full molecular basis or in the basis set of the chosen fragment. In the latter case, the same set of fragment orbitals can in the case of atomic fragments be used at different molecular geometries. One may even perform a geometry optimization, but only using the numerical gradient. When freezing orbitals the selected orbitals are first eliminated from the transformation to orthonormal basis, but then reintroduced in the backtransformation step. They will appear in the output with zero orbital eigenvalues. Note that when freezing orbitals the orbitals to be eliminated must be specified as well. The frozen orbitals must be a subset of the eliminated orbitals.
Fragments are defined with respect to the list of symmetry-independent atoms appearing in the DIRAC basis file: Consider the water molecule in the full C2*v*symmetry. Then there are two fragments: the oxygen atom and the H2 moiety. However, with no symmetry there will be three fragments: the oxygen atom and the two hydrogen atoms. At the moment there are no orthonormalization of fragments on different fragments and so in practice one should only use orbitals from one fragment.
Eliminate orbitals by projecting them out of the transformation matrix to orthonormal basis.
Arguments: Number of fragments (NPRJREF).
Then for each fragment read name PRJFIL of the coefficient file followed by the number of symmetry-independent nuclei in this fragment followed by an Specification of orbital strings of selected orbitals for each fermion irrep.
DO J = 1,NPRJREF
READ(LUCMD,'(A6)') PRJFIL(J)
READ(LUCMD,'(I6)') NPRJNUC(J)
READ(LUCMD,'(A72)') (VCPROJ(I,J),I=1,NFSYM)
ENDDO
Eliminated/frozen orbitals are given in the fragment basis. Note that the list of fragments is assumed to follow the list of symmetry independent nuclei in the DIRAC basis file.
Freeze orbitals (must only be used in conjunction with .PROJECT).
For each fermion irrep, give an Specification of orbital strings of orbitals to freeze.