RKS  The restricted KohnSham method will be used. This is the default for closedshell systems.  
UKS  The unrestricted KohnSham method will be used. This is the default for openshell systems.  
ROKS  The spinrestricted openshell KohnSham method will be used.  
ROKS option  The ROKS option string defines the ROKS parametrization according to Table 8. By default the Guest & Saunders parametrization [174] is used.  
CUKS  The constrained unrestricted KohnSham method will be used.  
MAX=Integer  Maximum number of SCF iterations. Default is 100.  
TOL=Real  MinMax SCF energy convergence criterion. Default is a.u.  
CDF=Real  Auxiliary density convergence criterion. Default is a.u.  
NOTIGHTEN  The SCF convergence criteria will not be adjusted during an optimization, frequency analysis or property calculation. 
(12) 
(13)  
(14)  
(15) 
Note that the ROKS orbital energies vary with respect to these parametrizations and, therefore, are not uniquely defined. In particular, the Aufbau principle for the doubly and singly occupied molecular orbital energy manifolds can be violated [179,180]. Moreover, ROKS orbital energies include additional constraints and cannot be compared with RKS or UKS orbital energies. Specifically, the SlaterJanak theorem [181,182] does not hold for ROKS calculations. For this reason the blockdiagonalized ROKS and MO energies and coefficients [183], also named semicanonical spin orbitals, are printed after the ROKS energy as shown in the following example output for a B3LYP/STO3G/GENA2* calculation of the triplet O ground state.
*** SCF CONVERGED *** MO COEFFICIENTS OF CYCLE 7 7 8 9 10 0.3794 0.0601 0.0601 0.4267 2.0000 1.0000 1.0000 0.0000 1 1 O 1s 0.0000 0.0000 0.0000 0.0917 2 1 O 2s 0.0000 0.0000 0.0000 0.5671 3 1 O 2py 0.6576 0.0822 0.7637 0.0000 4 1 O 2pz 0.0000 0.0000 0.0000 0.9484 5 1 O 2px 0.0374 0.7637 0.0822 0.0000 6 2 O 1s 0.0000 0.0000 0.0000 0.0917 7 2 O 2s 0.0000 0.0000 0.0000 0.5671 8 2 O 2py 0.6576 0.0822 0.7637 0.0000 9 2 O 2pz 0.0000 0.0000 0.0000 0.9484 10 2 O 2px 0.0374 0.7637 0.0822 0.0000 RANDOMIZED SCF GRID GENERATED IN 3 CYCLES REFERENCE VALUE OF S**2 FOR PURE SPIN STATE S(S+1): 2.0000 S**2 BEFORE SPIN PROJECTION: 2.0000 S**2 AFTER SPIN PROJECTION: 2.0000 ELECTRONIC CORE ENERGY = 260.328412563 ELECTRONIC COULOMB ENERGY = 101.099784250 ELECTRONIC HARTREE ENERGY = 159.228628313 EXCHANGE ENERGY = 16.350046036 CORRELATION ENERGY = 0.733801619 EXCHANGECORRELATION ENERGY = 17.083847655 ELECTRONIC SCF ENERGY = 176.312475968 NUCLEARREPULSION ENERGY = 28.059106307 TOTAL ENERGY = 148.253369660 BLOCK DIAGONAL ALPHA MO COEFFICIENTS 7 8 9 10 0.4028 0.1618 0.1618 0.4020 1.0000 1.0000 1.0000 0.0000 1 1 O 1s 0.0729 0.0000 0.0000 0.0917 2 1 O 2s 0.3516 0.0000 0.0000 0.5671 3 1 O 2py 0.0000 0.7148 0.2813 0.0000 4 1 O 2pz 0.6083 0.0000 0.0000 0.9484 5 1 O 2px 0.0000 0.2813 0.7148 0.0000 6 2 O 1s 0.0729 0.0000 0.0000 0.0917 7 2 O 2s 0.3516 0.0000 0.0000 0.5671 8 2 O 2py 0.0000 0.7148 0.2813 0.0000 9 2 O 2pz 0.6083 0.0000 0.0000 0.9484 10 2 O 2px 0.0000 0.2813 0.7148 0.0000 BLOCK DIAGONAL BETA MO COEFFICIENTS 7 8 9 10 0.3239 0.0415 0.0415 0.4515 1.0000 0.0000 0.0000 0.0000 1 1 O 1s 0.0000 0.0000 0.0000 0.0917 2 1 O 2s 0.0000 0.0000 0.0000 0.5671 3 1 O 2py 0.3948 0.7637 0.0825 0.0000 4 1 O 2pz 0.0000 0.0000 0.0000 0.9484 5 1 O 2px 0.5272 0.0825 0.7637 0.0000 6 2 O 1s 0.0000 0.0000 0.0000 0.0917 7 2 O 2s 0.0000 0.0000 0.0000 0.5671 8 2 O 2py 0.3948 0.7637 0.0825 0.0000 9 2 O 2pz 0.0000 0.0000 0.0000 0.9484 10 2 O 2px 0.5272 0.0825 0.7637 0.0000
The printing of the MO energies and coefficients is activated with PRINT MOS=710 (see Section 4.12.2). The block diagonal and MO energies and coefficients can be directly compared with their UKS counterparts. In deMon2k they are also used for ROKS perturbation theory calculations. As an alternative to ROKS the constrained unrestricted KohnSham (CUKS) method can be used for spinprojection [184] where the semicanonical orbitals are obtained directly. The convergence behavior of ROKS and CUKS calculations is generally different and it is advisable to switch among them to test convergence in problematic cases.
As a consequence of the variational fitting of the density [5,6], deMon2k can exploit a MinMax SCF procedure [185]. In a variational MinMax procedure, there is no strict convergence from above. Therefore, it is possible to obtain energies below the converged energy during the SCF iterations in deMon2k. Note that the printed SCF ERROR for an SCF cycle is the difference between upper and lower MinMax energy bounds and, thus, a direct measurement of how far the current SCF step is away from the convergence point.
The maximum number of SCF iterations is specified with the MAX option. With MAX=0 an "energy only" calculation with the molecular orbital coefficients from the restart file can be performed. No SCF iteration is done! The MAX=0 option automatically triggers GUESS RESTART (see 4.5.5) and, therefore, fails if no adequate restart file deMon.rst exists. The ordering and, thus, the occupation of the molecular orbitals in the restart file can be altered with the MOEXCHANGE keyword (see Section 4.4.3).
The SCF energy convergence criterion can be defined by the user with the TOL option. Such a userdefined SCF convergence criterion is valid for the first singlepoint SCF calculation. During a geometry optimization, however, the convergence criterion is automatically tightened according to the residual forces (see Table 9). However, if the userdefined convergence criterion is smaller than the automatictightening value, the userdefined value is used instead. If selfconsistent perturbation calculations are performed, the automatically requested SCF energy convergence criterion is Hartree. It too may be overridden by a smaller value with the TOL option.

The auxiliary density convergence criterion can be defined by the user with the CDF option. As with the userdefined energy convergence criterion, the userdefined auxiliary density convergence criterion holds for the first singlepoint SCF calculation and overrides the automatically determined CDF values (see Table 9) during the optimization if the userdefined value is smaller. If selfconsistent perturbation calculations are performed, the default auxiliary density convergence criterion is tightened to . Again, this value can be overridden by a smaller userdefined value with the CDF option. At SCF convergence both energy and auxiliary density convergence criteria are satisfied.
The option NOTIGHTEN disables the automatic tightening of the SCF convergence criteria according to the root mean square (RMS) gradient (as shown in Table 9). Thus, the SCF convergence tolerance is relaxed during geometry optimization and, therefore, SCF convergence failures are less likely. However, accuracy of the gradients is compromised and the structural optimization may not converge. Good practice, therefore, is to use NOTIGHTEN only at the beginning of the optimization or in combination with carefully tested userdefined TOL and CDF values. The NOTIGHTEN option also disables tightening of SCF convergence for perturbation calculations.