QQ-Interface (Quantics-QChem-Interface)

Purpose of Module

The Quantics-Qchem-Interface module connects the full quantum nonadiabatic wavefunction propagation code Quantics to the time-dependent density functional (TDDFT) module of the electronic structure program QChem. QChem provides analytic gradients, hessians and derivative couplings at TDDFT level. With this module it is possible to use the QChem TDDFT module for excited state direct dynamics calculations. Quantics will prepare the input file from a template, start QChem calculations whenever needed and will read the output of QChem. The QChem results are stored in the Quantics database and can be used in dynamics simulations. Due to the modular design of Quantics the TDDFT module of QChem can be used for all dynamics simulations, e.g. dd-vMCG (direct-dynamics with variational multi-configurational gaussians) or surface hopping simulations.


The module will be used to examine the nonadiabatic excited state dynamics of small to medium sized molecules. The TDDFT module of QChem allows to treat systems that are too large for efficient CASSCF calculations. Until today photoinduced dynamics simulations of such molecules were only possible using trajectory based algorithms. With Quantics a full quantum-mechanical description of the nuclear motion is possible.

Building and Testing

To use the module, get the latest version of Quantics from the repository and build it as usual. Moreover you have to have a running version of QChem installed on your system. An example calculation, simulating the photodissociation of water using 4 coupled states is added to the Quantics repository, the documentation of the example can be found at this link After you have copied the ‘water.inp’ and the ‘run_qchem’ files to your directory, you have specified the template for the electronic structure calculations and you performed the preparatory calculations, you can start the simulation with:

quantics -mnd water.inp

In the specific example, Quantics will search for a script called ‘run_qchem’ (specified in the input file) to start a QChem calculation. The file ‘run_qchem’ script is of course dependent on your system configuration and has to be adapted. For more information how to run the test simulation please refer to the Quantics documentation.

As vMCG dynamics is very sensitive to numerical issues it is possible, depending on your compiler and machine, that your results differ slightly from the provided reference values (in the order of a few percent), but the qualitative behavior of the results should be preserved.

Source Code

The source code for the QQ-Interface can be found within the Quantics software which can be downloaded via gitlab. You firstly need to make an account (at gitlab). The Quantics project has a private repository so you also need to be a member of the project to clone it into your computer, then type:

Within the Quantics program, explicit code for the QQ-Interface routine is located in the file ~/quantics/source/opfuncs/funcqchemmod.f90. Most changes can be found in the subroutines ‘ddqchem’ and ‘wrqchem’.