QMCChem

During the last years we have been actively developing the QMC=Chem quantum Monte Carlo code. This code was initially designed for massively parallel simulations, and uses the manager/worker model.

When the program starts its execution, the manager runs on the master node and spawns two other processes: a worker process and a data server. The worker is an efficient Fortran executable with minimal memory and disk space requirements (typically a few megabytes for each), where the only MPI communication is the broadcast of the input data (wave function parameters, initial positions in the 3N-space and random seed). The outline of the task of a worker is the following:

 while ( Running )
 {
    compute_a_block_of_data();
    Running = send_the_results_to_the_data_server();
 }

The data server is a socket server implemented in Python. When it receives the computed data of a worker, it replies to the worker the order given by the manager to compute another block or to stop. The received data is then stored in a database using an asynchronous I/O mechanism. The manager is always aware of the results computed by all the workers and controls the running/stopping state of the workers and the interaction of the user during the simulation.

QMC=Chem is very well suited to massive parallelism and cloud computing:

• All the implemented algorithms are CPU-bound
• All workers are totally independent
• The load balancing is optimal: the workers always work 100% of the time, independently of their respective CPU speeds
• The code was written to be as portable as possible: the manager is written in standard Python and the worker is written in standard Fortran.
• The network traffic is minimal and the amount of data transferred over the network can even be adjusted by the user
• The number of simultaneous worker nodes can be variable during a calculation
• Fault-tolerance is implemented
• The input and output data are not presented as traditional input files and output files. All the input and output data are stored in a database and an API is provided to access the data. This allows different forms of interaction of the user: scripts, graphical user interfaces, command-line tools, web interfaces, etc.

Current Features

Methods

• VMC
• DMC with stochastic reconfiguration
• Jastrow factor optimization
• CI coefficients optimization

Wave functions

• Single determinant
• Multi-determinant
• Nuclear cusp correction

Links

• Easy development with the IRPF90 tool.
• The access to input/output files is provided via an API produced by the Easy Fortran I/O library generator
• Documentation

Parallel speed-up curve

Number of computed blocks for the CuCl2 molecule. The simulation stops when the wall-time has reached 5 minutes. Each block is composed of 10 walkers realizing 2,000 VMC steps.

Features under development

Properties

• Molecular Forces
• Moments (dipole, quadrupole,...)
• Electron density

Practical aspects

• Web interface for input and output

Input file creation

The QMC=Chem input file can be created using the web interface. Upload a Q5Cost file or an output file from GAMESS, Gaussian or Molpro, and you will download the QMC=Chem input directory.

Papers related to the QMC=Chem code

• Quantum Monte Carlo for large chemical systems: Implementing efficient strategies for petascale platforms and beyond
• QMC=Chem: a quantum Monte Carlo program for large-scale simulations in chemistry at the petascale level and beyond
• Large-scale quantum Monte Carlo electronic structure calculations on the EGEE grid