Introduction
============

Running the relativistic calculation of excited states
using the time-dependent density functional theory method (TDDFT)
requires the following three steps

    SCF(guess)  -->  SCF(4c)  -->  TDDFT

Here the initial SCF(guess) step represents the self-consistent field (SCF)
procedure based on a non-relativistic or scalar-relativistic Hamiltonian and
serves primarily for obtaining a very good guess of initial molecular orbitals
for subsequent relativistic four-component (4c) calculations. In the second
step, the actual relativistic molecular orbitals are determined from the SCF
procedure based on the fully relativistic four-component Dirac--Coulomb
Hamiltonian, SCF(4c).  In the final TDDFT step, first N excitation energies are
determined.

Optionally one can also restart TDDFT step if the same number of excitation
energies is requested but with different setting. For example using Tamm-Dancoff
approximation (TDA) and ALDA approximation to the DFT kernel TDDFT(TDA/ALDA),
will provide smooth convergence, but adds unnecessary approximations to the
TDDFT expressions. These approximations can be then lifted by subsequent
calculation employing full kernel and TDA approximation TDDFT(TDA), or lifting
both TDA and ALDA approximations TDDFT(FULL).

    SCF(guess)  -->  SCF(4c)  -->  TDDFT(TDA/ALDA)  -->  TDDFT(TDA)  -->  TDDFT(FULL)

Note that restarting TDDFT is also useful when the procedure did not converge
(f.e. maxdim-subspace was too low), or when tighter convergence threshold
is required. 


Example of the workflow
=======================


Step 1: SCF(1c) 
------

1) running the scf calculation interactively:            

   respect  --scf  --inp=1c  --scratch=/path/to/my/scratch/directory



Step 2: SCF(4c) 
------

1) preparing data for a restart from SCF(guess):
   
   ln -s guess.50 4c.50


2) running the scf calculation interactively:            

   respect  --scf  --inp=4c  --restart  --scratch=/path/to/my/scratch/directory



Step 3: TDDFT(TDA/ALDA) 
------


1) running the TDDFT(TDA/ALDA) calculation interactively:            

   respect  --tddft  --inp=4c  --scratch=/path/to/my/scratch/directory



Step 4: TDDFT(TDA) 
------


1) preparing data for a restart from TDDFT(TDA/ALDA):
   
   ln -s 4c.50 4c-tda.50


2) running the TDDFT(TDA) calculation interactively:            

   respect  --tddft  --inp=4c-tda  --scratch=/path/to/my/scratch/directory



Step 5: TDDFT(FULL) 
------


   ln -s 4c-tda.50 4c-full.50
   respect  --tddft  --inp=4c-full  --scratch=/path/to/my/scratch/directory




Final notes
===========

The list of available input keywords can be found at www.respectprogram.org


