# Extending molecular simulation time scales: Parallel in time integrations for high-level quantum chemistry and complex force representations

## Abstract

Parallel in time simulation algorithms are presented and applied to conventional molecular dynamics (MD) and ab initio molecular dynamics (AIMD) models of realistic complexity. Assuming that a forward time integrator, f (e.g., Verlet algorithm), is available to propagate the system from time t{sub i} (trajectory positions and velocities x{sub i} = (r{sub i}, v{sub i})) to time t{sub i+1} (x{sub i+1}) by x{sub i+1} = f{sub i}(x{sub i}), the dynamics problem spanning an interval from t{sub 0}…t{sub M} can be transformed into a root finding problem, F(X) = [x{sub i} − f(x{sub (i−1})]{sub i} {sub =1,M} = 0, for the trajectory variables. The root finding problem is solved using a variety of root finding techniques, including quasi-Newton and preconditioned quasi-Newton schemes that are all unconditionally convergent. The algorithms are parallelized by assigning a processor to each time-step entry in the columns of F(X). The relation of this approach to other recently proposed parallel in time methods is discussed, and the effectiveness of various approaches to solving the root finding problem is tested. We demonstrate that more efficient dynamical models based on simplified interactions or coarsening time-steps provide preconditioners for the root finding problem. However, for MD and AIMD simulations, suchmore »

- Authors:

- Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 (United States)
- Department of Mathematics, University of Chicago, Chicago, Illinois 60637 (United States)
- Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093 (United States)

- Publication Date:

- OSTI Identifier:
- 22303583

- Resource Type:
- Journal Article

- Journal Name:
- Journal of Chemical Physics

- Additional Journal Information:
- Journal Volume: 139; Journal Issue: 7; Other Information: (c) 2013 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0021-9606

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; 97 MATHEMATICAL METHODS AND COMPUTING; ALGORITHMS; CONTROL; ELECTRONIC STRUCTURE; HYDROCHLORIC ACID; INTERACTIONS; MOLECULAR DYNAMICS METHOD; SIMULATION

### Citation Formats

```
Bylaska, Eric J., E-mail: Eric.Bylaska@pnnl.gov, Weare, Jonathan Q., E-mail: weare@uchicago.edu, and Weare, John H., E-mail: jweare@ucsd.edu.
```*Extending molecular simulation time scales: Parallel in time integrations for high-level quantum chemistry and complex force representations*. United States: N. p., 2013.
Web. doi:10.1063/1.4818328.

```
Bylaska, Eric J., E-mail: Eric.Bylaska@pnnl.gov, Weare, Jonathan Q., E-mail: weare@uchicago.edu, & Weare, John H., E-mail: jweare@ucsd.edu.
```*Extending molecular simulation time scales: Parallel in time integrations for high-level quantum chemistry and complex force representations*. United States. doi:10.1063/1.4818328.

```
Bylaska, Eric J., E-mail: Eric.Bylaska@pnnl.gov, Weare, Jonathan Q., E-mail: weare@uchicago.edu, and Weare, John H., E-mail: jweare@ucsd.edu. Wed .
"Extending molecular simulation time scales: Parallel in time integrations for high-level quantum chemistry and complex force representations". United States. doi:10.1063/1.4818328.
```

```
@article{osti_22303583,
```

title = {Extending molecular simulation time scales: Parallel in time integrations for high-level quantum chemistry and complex force representations},

author = {Bylaska, Eric J., E-mail: Eric.Bylaska@pnnl.gov and Weare, Jonathan Q., E-mail: weare@uchicago.edu and Weare, John H., E-mail: jweare@ucsd.edu},

abstractNote = {Parallel in time simulation algorithms are presented and applied to conventional molecular dynamics (MD) and ab initio molecular dynamics (AIMD) models of realistic complexity. Assuming that a forward time integrator, f (e.g., Verlet algorithm), is available to propagate the system from time t{sub i} (trajectory positions and velocities x{sub i} = (r{sub i}, v{sub i})) to time t{sub i+1} (x{sub i+1}) by x{sub i+1} = f{sub i}(x{sub i}), the dynamics problem spanning an interval from t{sub 0}…t{sub M} can be transformed into a root finding problem, F(X) = [x{sub i} − f(x{sub (i−1})]{sub i} {sub =1,M} = 0, for the trajectory variables. The root finding problem is solved using a variety of root finding techniques, including quasi-Newton and preconditioned quasi-Newton schemes that are all unconditionally convergent. The algorithms are parallelized by assigning a processor to each time-step entry in the columns of F(X). The relation of this approach to other recently proposed parallel in time methods is discussed, and the effectiveness of various approaches to solving the root finding problem is tested. We demonstrate that more efficient dynamical models based on simplified interactions or coarsening time-steps provide preconditioners for the root finding problem. However, for MD and AIMD simulations, such preconditioners are not required to obtain reasonable convergence and their cost must be considered in the performance of the algorithm. The parallel in time algorithms developed are tested by applying them to MD and AIMD simulations of size and complexity similar to those encountered in present day applications. These include a 1000 Si atom MD simulation using Stillinger-Weber potentials, and a HCl + 4H{sub 2}O AIMD simulation at the MP2 level. The maximum speedup ((serial execution time)/(parallel execution time) ) obtained by parallelizing the Stillinger-Weber MD simulation was nearly 3.0. For the AIMD MP2 simulations, the algorithms achieved speedups of up to 14.3. The parallel in time algorithms can be implemented in a distributed computing environment using very slow transmission control protocol/Internet protocol networks. Scripts written in Python that make calls to a precompiled quantum chemistry package (NWChem) are demonstrated to provide an actual speedup of 8.2 for a 2.5 ps AIMD simulation of HCl + 4H{sub 2}O at the MP2/6-31G* level. Implemented in this way these algorithms can be used for long time high-level AIMD simulations at a modest cost using machines connected by very slow networks such as WiFi, or in different time zones connected by the Internet. The algorithms can also be used with programs that are already parallel. Using these algorithms, we are able to reduce the cost of a MP2/6-311++G(2d,2p) simulation that had reached its maximum possible speedup in the parallelization of the electronic structure calculation from 32 s/time step to 6.9 s/time step.},

doi = {10.1063/1.4818328},

journal = {Journal of Chemical Physics},

issn = {0021-9606},

number = 7,

volume = 139,

place = {United States},

year = {2013},

month = {8}

}