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= RESP module for response properties based on HF and DFT =
== Keywords for general information ==
=== IPRT ===
Print level, >1 gives more information, >2 give more information about integral evaluations.

=== NPRT ===
=== CHCK ===
Check the interface with several external packages.

=== CTHRD ===
== Keyworks for processing excited-state information ==
=== METHOD ===
=1, ground state gradients; =2, excited-state calculations which will load TD-DFT output.

=== NFILES ===
Linked with '''istore''' value in TD-DFT input for loading output.

== Keyword for geometric derivatives ==
=== GEOM: NORDER ===
GEOM enables geometric derivatives, NORDER=1, gradient and fo-NACMEs; =2, hessian (not implemented yet.)

== Keywords for linear response calculations ==
=== LINE ===
Enable linear response

== REDUCED ==
Solve the response equation in its reduced form [(A-B)(A+B)-w2](X+Y)=Rvo+Rov (not preferred).

=== POLA: AOPER, BOPER, BFREQ ===
Polarizabiity: '''<<A;B>>(wB)''', where the operators A and B can be dipole (DIP), quadruple (QUA), SOC (HSO), EFG.

== Keywords for quadratic response calculations ==
=== QUAD ===
Enable quadratic response function (QRF) calculations

=== HYPE: AOPER, BOPER, BFREQ, COPER, CFREQ ===
Hyperpolarizability: '''<<A;B,C>>(wB,wC)'''

=== SINGLE:STATES ===
Single residue of QRF, STATES can be used to specify the number of states followed by a detailed specification via the triple (ifile,isym,istate).

=== DOUBLE: PAIRS ===
Double residue of QRF, PAIRS can be used to specify the number of pairs followed by a detailed specification via two triples (ifile,isym,istate,jfile,isym,jstate).

=== FNAC ===
First-order nonadiabatic couplings

=== NORESP ===
Neglect the response part of transition density matrix in DOUBLE and FNAC calculations (recommended)

== Keywords for finite difference calculations ==
=== FDIF ===
Enable finite difference calculations

=== STEP ===
followed by a real number for the step size, default 0.001 [unit].

=== BOHR ===
The default unit is angstrom, to use bohr. This keyword must be specified.

=== IGNORE ===
Ignore the recomputation of excitation energies for check consistency.

= Quick guides by examples =
The following examples give the minimal inputs for starting response calculations:

 1. [[Ground-state geometric derivatives]]
 1. [[Response properties based on response functions]]
  [[Excited-state properties based on analytic derivatives]]

=== Excited state dipole ===
=== Excited state gradient ===

[[Examples: first-order nonadiabatic couplings]]

=== Analytic derivative approach ===
=== Finite-difference approach ===
{{{
$COMPASS
Title
 nh3
Basis
 sto-3g
Geometry
 C                  0.00000000   -1.20809142   -1.14173975
 C                  0.00000000   -1.20797607    0.25342015
 C                  0.00000000    0.00000000    0.95085852
 C                 -0.00000000    1.20797607    0.25342015
 C                 -0.00000000    1.20809142   -1.14173975
 C                  0.00000000    0.00000000   -1.83922155
 H                  0.00000000   -2.16045397   -1.69142002
 H                  0.00000000   -2.16044427    0.80300713
 H                 -0.00000000    2.16044427    0.80300713
 H                 -0.00000000    2.16045397   -1.69142002
 H                  0.00000000    0.00000000   -2.93882555
 F                  0.00000000    0.00000000    2.30085848
End geometry
skeleton
group
c(1)
nosym
$END

$xuanyuan
direct
schwarz
$end

$scf
RHF
charge
0
spin
1
THRESHCONV
1.d-10 1.d-8
OPTSCR
1
iaufbau
0
$end

$tddft
imethod
1
isf
0
iexit
2
itda
1
idiag
1
istore
1
crit_e
1.d-10
crit_vec
1.d-8
lefteig
AOKXC
DirectGrid
$end

$resp
iprt
1
QUAD
FNAC
single
states
1
1 1 2
double
pairs
1
1 1 1 1 1 2
norder
1
method
2
nfiles
1
FDIF
step
0.001
ignore
1
noresp
$end
}}}
To use finite-difference, a script '''fdiff.py''' should be used as

{{{
./fbdiff.py run.sh input.inp > log
}}}
After the calculation is done, an output file '''input.out''' will present in the current directory. The '''log''' file saves the information during the calculations.

Note: If '''FDIF''' is omitted, the analytic calculation will be carried out by simply using the '''run.sh''' script.

[[Examples: pp-TDA based properties]]

= Some caveats before using this module =
=== dft ===
1. Thresholds in dft_prescreen.F90 have set very tight.

2. Keyword '''ixcfun''' in SCF allows to use original XC library (default) or XCFun lib (=1) by Ulf Ekström [http://www.admol.org/xcfun] in dft and tddft.

=== scf ===
1. Sgnfix: fix adjacent sign

2. iaufbau=3: fix ordering and sign with respect to the initial MOs.

3. Convergence