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= RESP module for response properties based on HF and DFT =

<<TableOfContents(4)>>

== 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|Response properties based on linear and quadratic response functions]]
 1. [[Excited-state properties based on analytic derivatives]]
 1. [[Examples: first-order nonadiabatic couplings|First-order nonadiabatic couplings]]
 1. [[Alternative of TD-DFT: particle-particle TDA (pp-TDA) based properties]]

= Some caveats before using this module =

=== dft ===

1. Thresholds in dft_prescreen.F90 have been 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. Tight convergence on density matrix is required.

2. '''sgnfix''': fix adjacent sign of MOs during SCF iterations

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


=== tddft ===

1. Tight convergence on eigenvectors

2. Keyword '''lefteig''' for storing left eigenvectors in TD-DFT

3. '''istore''' key the file number of TD-DFT calculations