Linear response calculations are the most efficient way to predict resonant electronic response frequencies and intensities. QChem can also propagate meanfield theories (HF, DFT) in realtime with and without dissipation and finite temperature effects. These methods timeevolve the Liouville equation for a oneparticle density matrix :
(6.58) 
These realtime methods are useful for simulating attosecond to picosecond timescales, density response to multiple or strong applied fields, and timescales of energy relaxation. Symmetry, nonsinglet densities, MetaGGAs and 5thrung functionals are not supported for realtime propagation, and nuclei are fixed. The CPU and memory costs of a realtime propagation scale with system size like a linear response calculation, and the walltime also grows linearly with the desired simulation time. The number of required Fockbuild steps can be estimated from the default electronic timestep, which is 0.02 atomic units ( fs). The realtime code exploits realtime sharedmemory parallelism, and the use of 8 or more cores (–nt 8) is suggested. The input file of a basic realtime propagation is relatively simple, but sophisticated jobs require additional input files, and generate additional output.
Example 6.178 QChem Basic RTTDDFT job for BH3.
$molecule 0 1 B 0.0000000000 0.0000000000 0.0000000000 H 1.1156099994 0.3220488680 0.2603090013 H 0.3323685610 1.1371841829 0.1113829664 H 0.7822059094 0.8144746635 0.3752943192 $end $rem SCF_GUESS core GEN_SCFMAN true RTTDDFT 1 Basis 321g thresh 10 scf_convergence 9 EXCHANGE b3lyp SYMMETRY false SYM_IGNORE TRUE SCF_ALGORITHM DIIS UNRESTRICTED FALSE MAX_SCF_CYCLES 200 $end
Before executing this job, a directory (/logs) must be made in the output directory of the job to collect most of the results of the propagation. By default, a realtime job begins from the ground state SCF density, applies a weak, brief (0.07 atomic time units) oscillating electric field to the yaxis of a molecule which excites a superposition of all ypolarized excited states, and outputs the resulting timedependent dipole moment, energy and other quantities to the logs directory. The parameters of the timedependent run are printed when the propagation begins. During the propagation the state of the molecule and propagation is summarized in the QChem output file, including estimates of the elapsed and remaining simulation walltime. Propagation stops when MaxIter time steps are exceeded, or will stop prematurely if the density matrix becomes nonphysical. The logs directory will be populated by several text, whitespace delimited tables by default. The ./logs/Pol.csv file is a table consisting of: (time (au), , , , Total energy, , the gap, the electronic energy, and two unused columns). The ./logs/Fourier_(xyz).csv file contains Fourier transforms of the dipole moment in each direction consisting of energy (eV), negative imaginary, and real parts respectively.
By adjusting the options of the propagation with a file TDSCF.prm, a much larger amount of output can be generated, including the electron and hole densities at all times as sequential MolDen files (*.mol) viewable with the free package Gabedit (gabedit.sourceforge.net). The *.mol files generated by the realtime code have fractional orbital occupation numbers, and do not render properly in viewers other than Gabedit to our knowledge. So long as the applied field is weak and has short duration the positions at which peaks appear in the Fourier are the same as a linearresponse TDDFTRPA job (note: not the same as a LRTDDFTTDA job), as shown in the sample. In an energyconserving job, the width of the peak is inversely proportional to the duration of the signal sample used to construct the Fourier transform. If the applied field pulse is long ( a.u.), or has strong intensity ( a.u.) nonlinear response can be studied. Nonlinear effects in realtime SCF are an area of active investigation and development. To make a rotationally averaged dipole absorption spectrum the three diagonal components of (the polarization which results in the direction after an electric field of magnitude in the direction) are required i.e:
(6.59) 
These elements of the polarizability can be made from three separate propagations.
Because of the large number of floating point arguments used to control a realtime job, a separate input file TDSCF.prm in the same directory as the qchem input file is used for parameters. The file is two columns of plain text. The first column is a string naming the parameter (which must match the case and spelling printed in the output exactly to be interpreted correctly), and the second column is a floating point number. The number must only be followed by a new line. Several inputs are interpreted as true=1 or false=0. The most useful parameters are discussed in this section. The code will signal if this file is not found and default values will be supplied instead.
Example 6.179 QChem Typical TDSCF.prm for a realtime B3LYP calculation.
dt 0.02 Stabilize 0 TCLOn 0 MaxIter 80000 ApplyImpulse 1 ApplyCw 0 FieldFreq 0.7 Tau 0.07 FieldAmplitude 0.001 ExDir 1.0 EyDir 1.0 EzDir 1.0 Print 0 StatusEvery 10 SaveDipoles 1 DipolesEvery 2 SavePopulations 0 SaveFockEnergies 0 WriteDensities 0 SaveEvery 500 FourierEvery 5000
TDSCF.prm Parameters 

Parameter String 
Explanation 
dt 
timestep (atomic units) >0.02 may be unstable. 
Stabilize 
Forces positive occupation numbers. 
MaxIter 
Maximum Timesteps 
ApplyImpulse 
0=No applied gaussian impulse 
ApplyCw 
0=No Continuous Wave, 1=Cosine applied field. 
FieldFreq 
Frequency of applied field (atomic u.) 
Tau 
Timevariance of Gaussian Impulse (atomic u.) 
FieldAmplitude 
Max. amplitude of field (atomic u.) 
ExDir 
Field Polarization Vector (xcomponent) 
EyDir 
Field Polarization Vector (ycomponent) 
EzDir 
Field Polarization Vector (zcomponent) 
Value makes print more debug output. 

StatusEvery 
Iterations between status in output file 
SaveDipoles 
0=No Pol.csv generated. 
DipolesEvery 
Iterations between samples of Dipole 
SavePopulations 
1=Saves diagonal of the density 
SaveFockEnergies 
1=Saves diagonal of the Fock matrix 
WriteDensities 
generates .mol files readable with gabedit 
SaveEvery 
Iterations between writing of all files in /logs 
FourierEvery 
Iterations between Fourier Transform 
FourierZoom 
A zoom parameter which controls resolution of FT. 
Any undocumented options not discussed above are not officially supported at this time.