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PROGRAM SUMMARY
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Title of program:
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BREIT_NO
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Catalogue identifier:
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ADLA
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Ref. in CPC:
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124(2000)247
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Distribution format: ** tar gzip file
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Operating system: ** Windows 97
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High speed store required:
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2MK words
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Peripherals Required: ** disc
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Number of lines in distributed program, including test data, etc:
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68547
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Programming language used: ** Fortran
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Computer: ** Pentium-based PCs

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Nature of physical problem:
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In many atomic processes involving inner atomic shells, the relaxation
of electron orbitals plays an important role. The relaxation can be
most naturally taken into consideration by using non-orthogonal orbitals
for the initial and final state. In atomic structure calculations, the
rate of convergence of the many-configuration expansion to an acceptable
accuracy may be much faster if the orbitals associated with different
configurations and terms are not necessarily required to be orthogonal.
This requires the evaluation of matrix elements of various operators
with respect to the non-orthogonal, one-particle orbitals. Initially,
these matrix elements can be expressed as weighted sums of radial
integrals, possibly multiplied by overlap integrals. The program
computes all coefficients of the radial integrals and the corresponding
overlap factors for matrix elements of the Breit-Pauli Hamiltonian, with
any amount of non-orthogonality between the involved orbitals.

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Method of solution
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At first, the configuration wave functions are expanded over the Slater
determinants [1], using a combination of vector coupling and fractional
parentage methods [2]. Then the coefficients of the radial integrals
and corresponding overlap factors are obtained from integration over all
spin and angular coordinates for the separate determinants [3]. Due to
the additional task of finding the determinant expansion, the method
used is more laborious than those based on the Fano-Racah technique [4]
and those widely used now for calculating the matrix elements with
orthogonal orbitals [5,6]. On the other hand, the present technique
admits a simple extension to the case of non-orthogonal orbitals in the
most general way. Besides, a considerable reduction of computation can
be achieved by reusing the previously obtained data allowed by the
present non-orthogonal technique.

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Restrictions on the complexity of the problem
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Any number of s, p, or d electrons are allowed in a shell, but no more
than two electrons or two holes in any shell of higher orbital angular
momentum. Only the shells outside the set of closed shells (core)
common to all configurations must be specified. The core orbitals are
assumed to be orthogonal to all others, and the interaction with the
core is included through a redefinition of the radial integrals. A
maximum 8 shells (in addition to the core) are allowed, with the maximum
number of electrons equal to 80. These restrictions can be eliminated
by changing the corresponding parameters and some format statements.

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Typical running time
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The typical running time is 0.001 to 10 seconds for a single matrix
element on a 200 Mhz Pentium-based PC, and depends crucially on
complexity of the involved configurations, primarily, on the size of
determinant expansions and the number of electrons.

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References
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[1] E.U. Condon and G.H. Shortley, The theory of atomic spectra (Cambridge Univ. Press, 1935). [2] O. Zatsarinny, Comput. Phys. Commun. 98 (1996) 235. [3] P.-O. Lowdin, Phys. Rev. 97 (1955) 1474. [4] U. Fano, Phys. Rev. A 140 (1965) 67. [5] A. Hibbert and C. Froese Fischer, Comput. Phys. Commun. 64 (1991) 417. [6] A. Hibbert, R. Glass and C. Froese Fischer, Comput. Phys. Commun. 64 (1991) 455.