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Subsections
8.34 MRCI
The
MRCI
program generates Multi Reference SDCI or ACPF[44]
wavefunctions. ACPF is a modification of the CPF[42]
method which allows more than one reference configuration. The program is
based on the Direct CI method[45],
and with the coupling coefficients generated with the Graphical Unitary Group
Approach[46]–[47].
(See program description for
GUGA).
If requested, MRCI computes matrix elements of those
oneelectron properties for which it can find integrals in the
ONEINT file. It also
generates natural orbitals that can be fed into
the property program to evaluate certain one electron properties.
The natural orbitals are also useful for Iterated Natural Orbital
(INO) calculations.
The MRCI code is a modification of an MRCI
program written by M. Blomberg and P. E. M. Siegbahn (Institute of Physics,
Stockholm University, Sweden), which has later been extensively modified
(P.Å. Malmqvist)
The program can calculate several eigenvectors simultaneously, not
necessarily those with lowest eigenvalue. However, in the ACPF case,
only one single eigenvector is possible.
The orbital space is divided into the following subspaces: Frozen,
Inactive, Active, Secondary, and Deleted orbitals. Within each
symmetry type, they follow this order.
 Frozen:
Frozen orbitals are always doubly
occupied, i.e., they are not correlated. Orbitals may be frozen
already in the integral integral transformation step, program
MOTRA, but can also be specified in the input to the
MRCI program. The former method is more efficient,
and has the effect that the frozen orbitals are effectively removed
from the subsequent
MRCI calculation.
 Inactive:
Inactive orbitals are doubly occupied
in all reference configurations, but excitations out of this orbital
space are allowed in the final CI wavefunction, i.e., they are
correlated but have two electrons in all reference configurations.
Restrictions may be applied to excitation from some inactive orbitals,
see keyword NoCorr in the GUGA input section.
 Active:
Active orbitals are those which may have
different occupation in different reference configurations.
Restrictions may be applied to occupation of some active orbitals,
see keyword OneOcc in the GUGA input section.
 Secondary:
This subspace is empty in all
reference configurations, but may be populated with up to two
electrons in the excited configurations. This subspace is not
explicitly specified, but consists of the orbitals which are left over
when other spaces are accounted for.
 Deleted:
This orbital subspace does not
participate in the CI wavefunction at all. Typically the 3s,4p,
components of 3d,4f, or orbitals that essentially describe core
correlation, are deleted. Similar to freezing, deleting can be done in
MOTRA,
which is more efficient, but also as input
specifications to the
MRCI program.
Since ordinarily the frozen and deleted orbitals were handled by
MOTRA
and the subdivision into inactive and
active orbitals were defined in
GUGA, the only
time one has to specify orbital spaces in the input to
MRCI
is when additional frozen or deleted orbitals are required without
recomputing the transformed integrals.
8.34.1 Dependencies
The program needs the coupling
coefficients generated by the program
GUGA and transformed one and twoelectron integrals
generated by the program
MOTRA.
8.34.2 Files
File  Contents

CIGUGA  Coupling coefficients from GUGA.

TRAINT*  Transformed twoelectron integrals from MOTRA.

TRAONE  Transformed oneelectron integrals from MOTRA.

ONEINT  Oneelectron property integrals from SEWARD.

MRCIVECT  Used for input only in restart case.

File  Contents

CIORBnn  One or more sets of natural orbitals, one for each CI root, where
nn stands for 01,02, etc.

MRCIVECT  CI vector, for later restart.

Note that these file names are the FORTRAN file names used by the program,
so they have to be mapped to the actual file names. This is usually done
automatically in the MOLCAS system. However, in the case of several
different numbered files
CIORBnn only the first will be defined as default,
with the FORTRAN file name
CIORB
used for
CIORB01 .
8.34.3 Input
This section describes the input to the
MRCI program in the MOLCAS program system, with
the program name:
&MRCI
Keyword  Meaning

TITLe  The line following this keyword is treated as title line

SDCI  This keyword is used to perform an ordinary MultiReference
Singles and Doubles CI, MRSDCI, calculation. This is the default
assumption of the program.
Note that SDCI and ACPF are mutually exclusive.

ACPF  This keyword tells the program to use the Average Coupled Pair
Functional, ACPF, when computing the energy and natural orbitals.
Note that SDCI and ACPF are mutually exclusive.

GVALue  The coefficient g which is used in the ACPF functional. The default
value is = 2.0/(Nr of correlated electrons).

NRROots  Specifies the number of CI roots (states) to be simultaneously
optimized. The default is 1.

ROOTs  Specifies which root(s) to converge to. These are defined as the
ordinal number of that eigenvector of the reference CI which is
used as start approximation. The default is the sequence 1,2,3
The values are entered on the next line(s). If the number of roots is
larger than 1, it must first have been entered using keyword NRROOTS.
The keywords ROOTS and SELECT are mutually exclusive.

SELEct  Another way of specifying the roots: instead of using ordinal
numbers, the roots selected will be those NRROOTS which have
largest projections in a selection space
which is specified on the next lines, as follows:
One line gives NSEL, the number of vectors used to define the
selection space. For each selection vector, program reads
the number of CSFs (NC), and # NC pairs of CSEL (text strings) and SSEL (coefficients).
The text string is composed of the
digits 0,1,2,3 and denotes the GUGA case numbers of the active
orbitals, defining uniquely a CSF belonging to the reference space.
The keywords ROOTS and SELECT are mutually exclusive.

RESTart  Restart the calculation from a previous calculation. No additional
input is required. The MRCIVECT file is required for restarted
calculations.

THRPrint  Threshold for printout of the wavefunction. All configurations with a
coefficient greater than this threshold are printed.
The default is 0.05. .

ECONvergence  Energy convergence threshold. The result is converged when the energy
of all roots has been lowered less than this threshold in the last
iteration. The default is 1.0d8.

PRINt  Print level of the program. Default is 5.

MAXIterations  Maximum number of iterations. Default 20. The maximum possible value is 49.

MXVEctors  Maximum number of trial vector pairs (CI+sigma) kept on
disk. Default is MAX(NRROOTS,10). It should never be
smaller than NRROOTS. A good value is 3*NRROOTS or more.

TRANsition  This keyword is relevant to a multiroot calculation. In addition
to properties, also
the transition matrix elements of various operators, for each pair
of wave functions, will be computed.

FROZen  Specify the number of orbitals to be frozen in
addition to the orbitals frozen in the integral transformation.
Default is 0 in all symmetries.

DELEted  Specify the number of orbitals to be deleted in
addition to the orbitals deleted in the integral transformation.
Default is 0 in all symmetries.

REFCi  Perform only reference CI.

PRORbitals  Threshold for printing natural orbitals. Only orbitals with occupation
number larger than this threshold appears in the printed output.
Default is 1.0d5.

&MRCI
Title
Water molecule. 1S frozen in transformation.
Sdci
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