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One of the central codes in MOLCAS is the RASSCF program, which
performs multiconfigurational SCF calculations. Both Complete Active Space
(CASSCF) and Restricted Active Space (RASSCF) SCF calculations can be performed
with the RASSCF program module .
An open shell Hartree-Fock calculation is not possible with the SCF
but it can be performed using the RASSCF module. An input listing for
a CASSCF calculation of water appears in Figure 6.7.
RASSCF requires orbital information of the system which can be
obtained in two ways. The LUMOrb indicates that the orbitals should be
taken from a user defined orbital file, which is copied to the internal file
INPORB. If this keyword is not given, the program will look for orbitals on the
runfile in the preference order: RASORB, SCFORB and
6.7 RASSCF -- A Multi Configurational
Self-Consistent Field Program
Sample input requesting the RASSCF module to calculate the eight-electrons-in-six-orbitals CASSCF energy of the second excited triplet state in the second symmetry group of a water molecule in C2v symmetry.
Title= The CASSCF energy of water is calculated using C2v symmetry. 2 3B2 state.
nActEl= 8 0 0
Inactive= 1 0 0 0; Ras2= 3 2 0 1
Symmetry= 2; Spin= 3
CIRoot= 1 2; 2
The TITLe performs the same function as in the previous MOLCAS modules. The keyword INACtive specifies the number of doubly occupied
orbitals in each symmetry that will not be included in the electron excitations
and thus remain doubly occupied throughout the calculation. A diagram of the
complete orbital space available in the RASSCF module is given in
In our calculation, we have placed the oxygen 1s orbital in the inactive
space using the INACtive keyword. The keyword FROZen can be
used, for example, on heavy atoms to reduce the Basis Set
Superposition Error (BSSE). The corresponding orbitals will then not be
optimized. The RAS2 keyword specifies the number of orbitals in each
symmetry to be included in the electron excitations with all possible
occupations allowable. Because the RAS1 and RAS3 spaces are
zero (not specified in the input in Figure 6.7) the
RASSCF calculation will produce a CASSCF wave function. The
RAS2 space is chosen to use all the orbitals available in each
symmetry (except the oxygen 1s orbital). The keyword NACTel
specifies the number of active electrons (8), maximum number of holes in the
Ras1 space (0) and the maximum number of electrons in the Ras3 space (0).
Using the keywords RAS1 and/or RAS3 to specify orbitals and
specifying none zero numbers of holes/electrons will produce a RASSCF wave
function.We are, therefore, performing an 8in6 CASSCF calculation of
Examples of types of wave functions obtainable using the RAS1 and RAS3 spaces in the RASSCF module.
||Number of holes
||Number of electrons
||in RAS1 orbitals
||in RAS3 orbitals
|Multi Reference SD-CI
|Multi Reference SD(T)-CI
There are a number of wave function types that can be performed by manipulating
the RAS1 and RAS3 spaces. Table 6.2 lists
a number of types obtainable. The first three are Configuration
Interaction (CI) wave functions of increasing magnitude culminating with a
Single, Double, Triples and Quadruples (SDTQ) CI. These can become
multi reference if the number of RAS2 orbitals is non-zero.
The last type provides some inclusion of the triples excitation by
allowing three holes in the RAS1 orbitals but save
computation cost by only allowing double excitations in the RAS3
The symmetry of the wave function is specified using the
SYMMetry keyword. It specifies the number of the symmetry
subgroup in the calculation. We have chosen the second symmetry
species, b2, for this calculation. We have also chosen the triplet
state using the keyword SPIN. The keyword CIROot has been
used to instruct RASSCF to find the second excited state in the
given symmetry and spin. This is achieved by specifying the number of roots,
1, the dimension of the small CI matrix which must be as large as the
highest required root and the number of the required second root.
Only for averaged calculations CIROot needs an additional line
containing the weight of the selected roots (unless equal weights are used for
RASSCF orbital space including keywords and electron occupancy ranges.
As an alternative to giving inactive and active orbital input we can use the
type index input on the INPORB and indicate there which type the
different orbitals should belong to: frozen (f), inactive (i), RAS1 (1), RAS2
(2), RAS3 (3), secondary (s), or deleted (d). This approach is very useful when the input
orbitals have been run through LUSCUS, which is used to select the
different subspaces. LUSCUS will relabel to orbitals according to the
users instructions and the corresponding orbital file ,GvOrb can be
linked as the INPORB in the RASSCF program without any
A level shift was included using the LEVShift keyword
to improve convergence of the calculation. In this case, the calculation
does not converge without the use of the level shift. It is advisable to
perform new calculations with a non-zero LEVShift value (the default
value is 0.5). Another possibility is to increase the maximum number of
iterations for the macro and the super-CI Davidson procedures
from the default values (200,100) using the keyword ITERations.
Sometimes convergence problems might appear when the wave function is
close to fulfill all the convergence criteria. An infrequent but possible
divergence might appear in a calculation starting from orbitals of an already
converged wave function, or in cases where the convergence thresholds
have been decreased below the default values.
Option TIGHt may be useful in those cases. It contains the
thresholds criteria for the Davidson diagonalization procedure. In situations
such as those described above it is recommended to decrease the first
parameter of TIGHt to a value lower than the default, for instance
The RASSCF section of the MOLCAS output contains similar
information to the SCF
output. Naturally, the fact that we have requested an excited state is
indicated in the output. In fact, both the lowest triplet state and the first
excited state or second root are documented including energies.
For both of these states the CI
configurations with a coefficient greater than 0.05 are printed along
with the partial electron distribution in the active space.
Figure 6.9 shows the relevant output for the second
root calculated. There are three configurations with a CI-coefficient
larger than 0.05 and two with very much larger values. The number of the
configuration is given in the first column and the CI-coefficient and
weight are given in the last two columns. The electron occupation of the
orbitals of the first symmetry for each configuration is given under the
`111' using `2' for a fully occupied orbital and `u'
for a singly occupied orbital containing an electron with an up spin.
The down spin electrons are represented with a `d'. The occupation
numbers of the active space for each symmetry is given below the contributing
configurations. It is important to remember that the active orbitals are
not ordered by any type of criterion within the active space.
RASSCF portion of output relating to CI configurations and electron occupation of natural orbitals.
printout of CI-coefficients larger than .05 for root 2
conf/sym 111 22 4 Coeff Weight
3 22u u0 2 .64031 .40999
4 22u 0u 2 .07674 .00589
13 2u0 2u 2 -.75133 .56450
14 2u0 u2 2 .06193 .00384
19 udu 2u 2 .06489 .00421
Natural orbitals and occupation numbers for root 2
sym 1: 1.986957 1.416217 .437262
sym 2: 1.567238 .594658
sym 4: 1.997668
The molecular orbitals are displayed in a similar fashion to the
SCF section of the output except that the energies of the
active orbitals are not defined and therefore are displayed as zero and
the electron occupancies are those calculated by the RASSCF
module. In a state average calculation (more than one root calculated),
the MOs will be the natural orbitals corresponding to the state
averaged density matrix (called pseudo-natural orbitals) and the occupation
numbers will be the corresponding eigenvalues. Natural orbital occupation
numbers for each state are printed as shown in Figure 6.9, but
the MOs specific to a given state are not shown in the output. They are,
however, available in the JOBIPH file. A number of molecular
properties are also computed for the requested electronic state in a similar
fashion to the SCF module.
6.7.2 Storing and Reading RASSCF Orbitals and Wave Functions
Part of the information stored in the RASSCF output file, JOBIPH,
for instance the molecular orbitals and occupation numbers can be also found
in an editable file named RASORB, which is automatically generated by
RASSCF. In case more than one root is used the natural orbitals are
also stored in files RASORB.1, RASORB.2, etc, up to ten. In such
cases the file RASORB contains the averaged orbitals. If more roots
are used the files can be generated using the OUTOrbitals keyword.
The type of orbital produced can be either AVERaged,
NATUral, CANOnical or SPIN (keywords) orbitals.
The OUTOrbitals keyword, combined with the ORBOnly keyword,
can be used to read the JOBIPH file and produce
an orbital file, RASORB, which can be read by a subsequent
RASSCF calculation using the same input section.
The formatted RASORB file is useful to operate on the orbitals in order
to obtain appropriate trial orbitals for a subsequent RASSCF
calculation. In particular the type index can be changed
directly in the file if the RASSCF program has converged to a solution
with wrong orbitals in the active space. The RASSCF program
will, however, automatically place the orbital files from the calculation in the
user's home directory under the name $Project.RasOrb, etc. In
calculations with spin different from zero the program will also produce the
spin orbital files $Project.SpdOrb1, etc for each state. These orbitals
can be used by the program LUSCUS to produce spin densities.
|SYMMetry||Symmetry of the wave function (according to GATEWAY)
(1 to 8)
|NACTel||Three numbers: Total number of active electrons, holes in Ras1, particles in Ras3
|INACtive||By symmetry: doubly occupied orbitals
|RAS1||By symmetry: Orbitals in space Ras1 (RASSCF)
|RAS2||By symmetry: Orbitals in space Ras1 (CASSCF and RASSCF)
|RAS3||By symmetry: Orbitals in space Ras1 (RASSCF)
|CIROot||Three numbers: number of CI roots, dimension of the CI matrix, relative weights
|LUMORB/FILEORB||use definition of active space from Orbital file
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Previous: 6.6 MBPT2