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= Xuanyuan =


<<TableOfContents(4)>>

{{{
Xuanyuan is used to calculate one electron and two electron integrals. It is named after Chinese ancestor Xuanyuan Huangdi.
}}}


== General keywords ==
=== Direct ===
{{{#!wiki
Ask for integral direct calculations. 
}}}

=== Schwarz ===
{{{#!wiki
Used with direct, ask for Schwarz equality prescreening.
}}}
Examples:
{{{
$xuanyuan
Direct
Schwarz
$end
}}}


=== Maxmem ===
{{{#!wiki
Set maximum memory used in integral calculation.   Unit can be MW and GW, i.e. Mega Words and Giga Words
}}}

Examples: 
{{{
$xuanyuan
Maxmem
  512MW 
$end
}}}

=== RS ===
{{{#!wiki
  Range separation ERIs required. 
  No default value. Suggested value: 0.33.
}}}

Examples: 
{{{
$xuanyuan
Rs
 0.33
$end
}}}

=== Scalar & Heff ===
{{{#!wiki
Scalar is a keyword to turn on scalar relativistic effects using sf-X2C (Heff=3) by default.

Other options for Heff are (0, nonrelativistic; 1, sf-ZORA; 2, sf-IORA; 3/4, sf-X2C; 5, sf-X2C+so-DKH3 (spin-free); 21, sf-X2C with gradients)
}}}

Examples:
{{{
$xuanyuan
scalar
heff
3
$end
}}}

=== Socint & Hsoc ===
{{{#!wiki
Socint is a keyword to turn on soc integral calculations in post-SCF steps.
Default option for hsoc is 0 (only 1e-soc int). 

Other options are used in soint_util/somf2e.F90 for choosing different combinations of so1e and so2e operators.

0 so-1e

1 so-1e + somf (two-electron spin-orbit interaction is included via an effective fock operator)

2 so-1e + somf-1c (one-center approximation)

3 so-1e + somf-1c / no soo (turn off spin-other-orbit contributions)

4 so-1e + somf-1c / no soo + WSO_XC (use dft xc functional as soo part)

5 so-1e + somf-1c / no soo + WSO_XC(-2x: following Neese's paper scale dft part by -2 to mimic soo part)

These options plus 10 gives the operators in BP approximations. In practice, hsoc=1 is the most accurate, and hsoc=2 is preferred for large molecules.

Note if heff=5, then the one-electron part will be calculated in xuanyuan and stored in disc for so-DKH3 type one-electron spin-orbit term.
The accuracy of such operator requires further tests.

}}}

Examples:
{{{
$xuanyuan
scalar
heff
3
socint
hsoc
2
$end
}}}

=== Nuclear & Inuc ===
{{{#!wiki
Inuc defines the nuclear charge distribution used in the V and pVp integrals, which can be -1 for point charge model (debug only), 0 for point charge model (default), 1 for finite nucleus model by an s-type Gaussian function, and other finite nucleus models (N.Y.I.).

For Za < 110, the nuclear charge radii are taken from Visscher and Dyall, At. Data and Nucl. Data Tables 67, 207, 1997 (in a.u).

For Za >= 110, the nuclear charge radius is 0.57 + 0.836 * A^1/3^ (in fm), where the isotope mass number A is estimated by Za according to the relationship A(Za) = 0.004467 * Za^2^ + 2.163 * Za - 1.168. See Appendix A in D. Andrae, Phys. Rep. 336, 414, 2000, and D. Andrae, Nuclear charge density distributions in quantum chemistry, in Relativistic Electronic Structure Theory, Part 1: Fundamentals, P. Schwerdtfeger Ed., Theoretical and Computational Chemistry, Vol. 11, Elsevier, 2002.
}}}

=== Cholesky ===
{{{#!wiki
  The following line contains a string and a float number.
  Set method and threshold of ERI Cholesky decomposition.
  S-CD for standard CD. 1c-CD for one center Cholesky decomposition.
}}}

Examples:
{{{
$xuanyuan
Cholesky
  S-CD 1.d-5
$end
}}}

== Expert keywords ==

=== NoCheck ===
{{{#!wiki
For Heff=21 only: check inverse variational collapse (IVC; see Liu and Kutzelnigg, J. Chem. Phys. 126, 114107, 2007). Stop (0; default) or not (1) in the case of IVC.

IVC may lead to numerical instability, which may be serious in geometry optimization.
}}}

=== NRDebug ===
{{{#!wiki
In relativistic calculations, use a C-light of 10^8 to reproduce non-relativistic results (for debug only).
}}}

=== Keyword3 ===
{{{#!wiki
xxx
}}}

=== Keyword4 ===
{{{#!wiki
xxx
}}}

= Depend Files =

|| Filename || Description || Format ||
||          ||             ||        ||

= Examples =