The ChIMES Calculator

Overview

ChIMES is a reactive explicitly many-bodied machine learned interatomic potential (ML-IAP) for which interactions are computed on the basis of atom clusters. For example, the total ChIMES energy is given as:

\begin{eqnarray} E_{n_\mathrm{B}} = \sum_{i_1} ^{n_a} {}^{1}\!E_{i_1} + \sum_{i_1>i_2} ^{n_a} {}^{2}\!E_{i_1i_2} + \sum_{i_1>i_2>i_3}^{n_a} {}^{3}\!E_{i_1i_2i_3} + \dots + \sum_{i_1>i_2\dots >i_{n\mathrm{B}-1}>i_{n\mathrm{B}}}^{n_a} {}^{n}\!E_{i_1i_2\dots i_n} \end{eqnarray}

where \(E_{n_\mathrm{B}}\) is the total ChIMES system energy, \(n_{\mathrm{B}}\) is the maximum bodiedness, \({}^{n}\!E_{i_1i_2\dots i_n}\) is the \(n\)-body ChIMES energy for a given set of \(n\) atoms with indices \(i = {i_1, i_2, \dots , i_n}\), and \(n_a\) is the total number of atoms in the system. In the ChIMES framework, single-body energies are constant values and \(n\)-body energies are constructed from the product of polynomials of transformed atom pair distances. Thus, a 2-body interaction would involve a single pair, \(ij\), while a three-body interaction would involve three pairs, \(ij, ik,\) and \(jk\), a 4-body interaction would involve \(4\choose 2\) pairs, and so on. Currently, the ChIMES calculator supports up to 4-body interactions.

For further details of the ChIMES ML-IAP equations, the reader is referred to the following. For a complete set of ChIMES references, see Citing ChIMES.

1. R.K. Lindsey*, L.E. Fried, N. Goldman, J. Chem. Theory Comput., 13 6222 (2017). (link)

2. R.K. Lindsey*, L.E. Fried, N. Goldman, J. Chem. Theory Comput. 15 436 (2019). (link)

3. R.K. Lindsey*, N. Goldman, L.E. Fried, S. Bastea, J. Chem. Phys. 153 054103 (2020). (link)

4. R.K. Lindsey*, L.E. Fried, N. Goldman, S. Bastea, J. Chem. Phys. 153 134117 (2020). (link)

Corresponding authors are indicated with an asterisk (*).

Sections

• ChIMES Calculator

• C API

• Fortran API

• Python API

• Implementation Examples

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The ChIMES Calculator

The ChIMES Calculator source files are located in chimesFF/src. To use in a C++ code, simply #include "chimescalc.h" in the target code and instantiate a chimesFF object. As described in greater detail below, chimesFF objects take information on individual atom clusters and provide the corresponding ChIMES energy, stress tensor, and forces. Any such code must at least include the following operations, in order:

int my_rank = 0;
chimesFF my_chimesFF_object;      // Instantiate
my_chimesFF_object.init(my_rank); // Set MPI rank (replace with zero if used in serial code)
my_chimesFF_object.read_parameters("my_parameter_file");

Note that the ChIMES calculator chimesFF class provides users with the following functions:

Return Type	Name	Arguments and Description
void	init	Type Description int MPI rank Set the MPI rank. With the exception of error messages, the ChIMES calculator will only print output for rank 0.
void	read_parameters	Type Description string Parameter file Read the chimes parameter file.
void	set_atomtypes	Type Description vector<string> List of atom types defined by parameter file (updated by function) Update the input vector with atom types in the parameter file.
double	max_cutoff_2B	Type Description bool Flag: If true, prints largest 2-body cutoff Returns the maximum 2-body outer cutoff distance.
double	max_cutoff_3B	Type Description bool Flag: If true, prints largest 3-body cutoff Returns the maximum 3-body outer cutoff distance.
double	max_cutoff_4B	Type Description bool Flag: If true, prints largest 4-body cutoff Returns the maximum 4-body outer cutoff distance.
void	compute_1B	Type Description int Atom type index double Energy (updated) Update energy with the single atom contribution.
void	compute_2B	Type Description double Distance between two atoms, i and j vector<double> Distance vector components for each atom vector<int> Type indices for atoms i and j vector<vector<double* > > Force pointer ([atom index (out of 2)][component index (i.e. fx=0, fy=1, fz=3)]) (contents updated by function) vector<double*> Stress tensor pointer ([s_xx, s_xy, s_xz, s_yx, s_yy, s_yz, s_zx, s_zy, s_zz]) (contents updated by function) double Energy (updated by function) Update the force pointer, stress tensor pointer, and energy with the two-atom contribution.
void	compute_3B	Type Description vector<double> Distances between three atoms, ij, ik, and jk vector<vector<double> > Distance vector components for each atom vector<int> Type indices for atoms i, j and k vector<vector<double* > > Force pointer ([atom index (out of 3)][component index (i.e. fx=0, fy=1, fz=3)]) (contents updated by function) vector<double*> Stress tensor pointer ([s_xx, s_xy, s_xz, s_yx, s_yy, s_yz, s_zx, s_zy, s_zz]) (contents updated by function) double Energy (updated by function) Update the force pointer, stress tensor pointer, and energy with the three-atom contribution.
void	compute_4B	Type Description vector<double> Distance between four atoms, ij, ik, il, jk, jl, and kl vector<vector<double> > Distance vector components for each atom vector<int> Type indices for atoms i, j, k and l vector<vector<double* > > Force pointer ([atom index (out of 4)][component index (i.e. fx=0, fy=1, fz=3)]) (contents updated by function) vector<double*> Stress tensor pointer ([s_xx, s_xy, s_xz, s_yx, s_yy, s_yz, s_zx, s_zy, s_zz]) (contents updated by function) double Energy (updated by function) Update the force pointer, stress tensor pointer, and energy with the four-atom contribution.

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The C API

The C API (chimescalc_C*) is located in chimesFF/api. This wrapper provides C style name mangling and creates a set of C-style wrapper functions. The latter are needed for compatibility with std::vector which is heavily used in chimesFF and not supported in most other languages. Any C code attempting to use the ChIMES Calculator should #include "chimescalc_C.h" and at least include the following operations, in order:

int my_rank = 0;
set_chimes();         // Instantiate
init_chimes(my_rank); // Set MPI rank (replace with zero if used in serial code)
chimes_read_params("my_parameter_file");

For additional information on compiling, see Implementation Examples.

Note that the ChIMES calculator chimescalc_C API provides users with the following functions:

Return Type	Name	Arguments and Description
void	set_chimes	No arguments. Instantiates a pointer to a chimesFF object.
void	init_chimes	Type Description int MPI rank Set the MPI rank. With the exception of error messages, the ChIMES calculator will only print output for rank 0.
void	chimes_read_params	Type Description char* Parameter file Read the chimes parameter file.
int	get_chimes_2b_order	No arguments. Returns the two body order set by the parameter file.
int	get_chimes_3b_order	No arguments. Returns the three body order set by the parameter file.
int	get_chimes_4b_order	No arguments. Returns the four body order set by the parameter file.
double	get_chimes_max_2b_cutoff	No arguments. Returns the two body maximum outer cutoff set by the parameter file.
double	get_chimes_max_3b_cutoff	No arguments. Returns the three body maximum outer cutoff set by the parameter file.
double	get_chimes_max_4b_cutoff	No arguments. Returns the four body maximum outer cutoff set by the parameter file.
void	chimes_compute_2b_props	Type Description double Distance between two atoms, i and j double array Distance vector components for each atom char* array Atom types for atoms i and j double array Forces for atoms i and j ([atom index (out of 2)][component index (i.e. fx=0, fy=1, fz=3)]) (contents updated by function) double array Stress tensor ([s_xx, s_xy, s_xz, s_yx, s_yy, s_yz, s_zx, s_zy, s_zz]) (contents updated by function) double* Energy (updated by function) Update the force, stress tensor, and energy with the two-atom contribution.
void	chimes_compute_3b_props	Type Description double array Distances between three atoms, ij, ik, and jk double array Distance vector components for each atom [atom][x, y, or z component] char* array Atom types for atoms i, j and k double array Forces for atoms i, j, and k ([atom index (out of 3)][component index (i.e. fx=0, fy=1, fz=3)]) (contents updated by function) double array Stress tensor ([s_xx, s_xy, s_xz, s_yx, s_yy, s_yz, s_zx, s_zy, s_zz]) (contents updated by function) double* Energy (updated by function) Update the force, stress tensor, and energy with the three-atom contribution.
void	chimes_compute_4b_props	Type Description double array Distances between four atoms, ij, ik, il, jk, jl, and kl double array Distance vector components for each atom [atom][x, y, or z component] char* array Atom types for atoms i, j, k and l double array Forces for atoms i, j, k, and l ([atom index (out of 4)][component index (i.e. fx=0, fy=1, fz=3)]) (contents updated by function) double array Stress tensor ([s_xx, s_xy, s_xz, s_yx, s_yy, s_yz, s_zx, s_zy, s_zz]) (contents updated by function) double* Energy (updated by function) Update the force, stress tensor, and energy with the four-atom contribution.
void	chimes_compute_2b_props_fromf90	Type Description double* Distance between two atoms, i and j double array Distance vector components for each atom char* Type for atom i char* Type for atom j double array Forces for atoms i and j ([atom index (out of 2)][component index (i.e. fx=0, fy=1, fz=3)]) (contents updated by function) double array Stress tensor ([s_xx, s_xy, s_xz, s_yx, s_yy, s_yz, s_zx, s_zy, s_zz]) (contents updated by function) double* Energy (updated by function) For calls from a Fortran code. Update the force, stress tensor, and energy with the two-atom contribution.
void	chimes_compute_3b_props_fromf90	Type Description double Distances between three atoms, ij, ik, and jk double array Distance vector components for each atom [atom][x, y, or z component] char* Type for atom i char* Type for atom j char* Type for atom k double array Forces for atoms i, j, and k ([atom index (out of 3)][component index (i.e. fx=0, fy=1, fz=3)]) (contents updated by function) double array Stress tensor ([s_xx, s_xy, s_xz, s_yx, s_yy, s_yz, s_zx, s_zy, s_zz]) (contents updated by function) double* Energy (updated by function) For calls from a Fortran code. Update the force, stress tensor, and energy with the three-atom contribution.
void	chimes_compute_4b_props_fromf90	Type Description double Distances between four atoms, ij, ik, il, jk, jl, and kl double array Distance vector components for each atom [atom][x, y, or z component] char* Type for atom i char* Type for atom j char* Type for atom k char* Type for atom l double array Forces for atoms i, j, k, and l ([atom index (out of 4)][component index (i.e. fx=0, fy=1, fz=3)]) (contents updated by function) double array Stress tensor ([s_xx, s_xy, s_xz, s_yx, s_yy, s_yz, s_zx, s_zy, s_zz]) (contents updated by function) double* Energy (updated by function) For calls from a Fortran code. Update the force, stress tensor, and energy with the four-atom contribution.

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The Fortran API

The Fortran API (chimescalc_F*) is located in chimesFF/api. This wrapper enables access to chimesFF functions through the C API and handles other details like differences in array storage order.

Any Fortran code attempting to use the ChIMES Calculator should use chimescalc and at least include the following operations, in order:

integer(C_int) :: my_rank
call f_set_chimes()         ! Instantiate
call f_init_chimes(my_rank) ! Set MPI rank (replace with zero if used in serial code)
call f_chimes_read_params(string2Cstring("my_parameter_file"))

For additional information on compiling, see Implementation Examples.

Note that the ChIMES calculator chimescalc_F API provides users with the following functions:

Return Type	Name	Arguments and Description
none	f_chimes_compute_2b_props_fromf90	Type Description C_double Distance between two atoms, i and j C_double array Distance vector components for each atom C_char Type for atom i C_char Type for atom j C_double array Forces for atoms i and j ([atom index (out of 2)][component index (i.e. fx=0, fy=1, fz=3)]) (contents updated by function) C_double array Stress tensor ([s_xx, s_xy, s_xz, s_yx, s_yy, s_yz, s_zx, s_zy, s_zz]) (contents updated by function) C_double Energy (updated by function) Update the force, stress tensor, and energy with the two-atom contribution.
none	f_chimes_compute_3b_props_fromf90	Type Description C_double array Distances between three atoms, ij, ik, and jk C_double array Distance vector components for each atom C_char Type for atom i C_char Type for atom j C_char Type for atom k C_double array Forces for atoms i, j, and k ([atom index (out of 3)][component index (i.e. fx=0, fy=1, fz=3)]) (contents updated by function) C_double array Stress tensor ([s_xx, s_xy, s_xz, s_yx, s_yy, s_yz, s_zx, s_zy, s_zz]) (contents updated by function) C_double Energy (updated by function) Update the force, stress tensor, and energy with the three-atom contribution.
none	f_chimes_compute_4b_props_fromf90	Type Description C_double array Distances between four atoms, ij, ik, il, jk, jl, and C_double array Distance vector components for each atom C_char Type for atom i C_char Type for atom j C_char Type for atom k C_char Type for atom l C_double array Forces for atoms i, j, k, and l ([atom index (out of 2)][component index (i.e. fx=0, fy=1, fz=3)]) (contents updated by function) C_double array Stress tensor ([s_xx, s_xy, s_xz, s_yx, s_yy, s_yz, s_zx, s_zy, s_zz]) (contents updated by function) C_double Energy (updated by function) Update the force, stress tensor, and energy with the four-atom contribution.
none	f_set_chimes	No arguments. Instantiates a pointer to a chimesFF object.
none	f_init_chimes	Type Description int MPI rank Set the MPI rank. With the exception of error messages, the ChIMES calculator will only print output for rank 0.
none	f_chimes_read_params	Type Description C_char Parameter file Read the chimes parameter file.
C_int	f_get_chimes_2b_order	No arguments. Returns the two body order set by the parameter file.
C_int	f_get_chimes_3b_order	No arguments. Returns the three body order set by the parameter file.
C_int	f_get_chimes_4b_order	No arguments. Returns the four body order set by the parameter file.
C_double	f_get_chimes_max_2b_cutoff	No arguments. Returns the two body maximum outer cutoff.
C_double	f_get_chimes_max_3b_cutoff	No arguments. Returns the three body maximum outer cutoff.
C_double	f_get_chimes_max_4b_cutoff	No arguments. Returns the four body maximum outer cutoff.
C_string	string2Cstring	Type Description string Any text Converts a Fortran string to a C_string

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The Python API

The Python API (chimescalc_py*) is located in chimesFF/api. Like the Fortran API, this wrapper enables access to chimesFF functions through the C API, via ctypes.

Any python code attempting to use the ChIMES Calculator should import chimescalc_py and at least include the following operations, in order:

chimescalc_py.chimes_wrapper = chimescalc_py.init_chimes_wrapper("lib-C_wrapper-serial_interface.so") # Associate the wrapper with a compiled C API library file
chimescalc_py.set_chimes()  # Instantiate
chimescalc_py.init_chimes() # If run with MPI, an integer MPI rank can be passed to this function. By default, assumes rank = 0
chimescalc_py.read_params("my_parameter_file")

For additional information on compiling (i.e. generation of lib-C_wrapper-serial_interface.so), see Implementation Examples.

Note that the ChIMES calculator chimescalc_py API provides users with the following functions:

Return Type	Name	Arguments and Description
ctypes	init_chimes_wrapper	Type Description str C-wrapper library name (i.e. “lib-C_wrapper-serial_interface.so”)
none	set_chimes	No arguments. Instantiates a pointer to a chimesFF object.
none	init_chimes	Type Description int MPI rank (optional parameter) Set the MPI rank. With the exception of error messages, the ChIMES calculator will only print output for rank 0.
none	read_params	Type Description str Parameter file
float	get_chimes_max_2b_cutoff	No arguments. Returns the two body order set by the parameter file.
float	get_chimes_max_2b_cutoff	No arguments. Returns the three body order set by the parameter file.
float	get_chimes_max_2b_cutoff	No arguments. Returns the four body order set by the parameter file.
int	get_chimes_2b_order	No arguments. Returns the two body maximum outer cutoff.
int	get_chimes_3b_order	No arguments. Returns the three body maximum outer cutoff.
int	get_chimes_4b_order	No arguments. Returns the four body maximum outer cutoff.
none	chimes_compute_2b_props	Type Description float Distances between atoms i and j float list Distance vector components for each atom str list Types for atom i and j float list Forces for atoms i, and j ([atom index (out of 2)][component index (i.e. fx=0, fy=1, fz=3)]) (contents updated by function) float list Stress tensor ([s_xx, s_xy, s_xz, s_yx, s_yy, s_yz, s_zx, s_zy, s_zz]) (contents updated by function) float Energy (updated by function) Update the force, stress tensor, and energy with the two-atom contribution.
none	chimes_compute_3b_props	Type Description float list Distances between three atoms, ij, ik, and jk float list Distance vector components for each atom str list Types for atom i, j, and k float list Forces for atoms i, j, and k ([atom index (out of 3)][component index (i.e. fx=0, fy=1, fz=3)]) (contents updated by function) float list Stress tensor ([s_xx, s_xy, s_xz, s_yx, s_yy, s_yz, s_zx, s_zy, s_zz]) (contents updated by function) float Energy (updated by function) Update the force, stress tensor, and energy with the three-atom contribution.
none	chimes_compute_4b_props	Type Description float list Distances between four atoms, ij, ik, il, jk, jl, and kl float list Distance vector components for each atom str list Types for atom i, j, k, and l float list Forces for atoms i, j, k, and l ([atom index (out of 4)][component index (i.e. fx=0, fy=1, fz=3)]) (contents updated by function) float list Stress tensor ([s_xx, s_xy, s_xz, s_yx, s_yy, s_yz, s_zx, s_zy, s_zz]) (contents updated by function) float Energy (updated by function) Update the force, stress tensor, and energy with the four-atom contribution.

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Implementation Examples

The following codes demonstrates how chimesFF.{h,cpp} can be used to obtain the overall stress tensor, energy, and per-atom forces for a given system configuration using C, C++ Fortran, and Python. See the main.* files in each corresponding subdirectory of chimesFF/examples for further implementation details. Note that sample system configurations (i.e. *xyz files) and parameter files can be found in serial_interface/test/configurations and serial_interface/test/force_fields, respectively. For user generated tests, note that *.xyz files must provide lattice vectors in the comment line, e.g. lx 0.0 0.0 0.0 ly 0.0 0.0 0.0 lz. Click here for an overview of ChIMES units.

Note

All implementation examples are intended to be run on Unix-based systems (e.g. Linux, OSX).

Warning

These codes are for demonstrative purposes only and come with no guarantees.

Note

All example executables can be compiled at once via ./install.sh from the chimes_calculator base directory, and similarly uninstalled via ./uninstall.sh. However, the examples below compile via the user-generated Makefiles located in each examples subdirectory, for demonstrative purposes.

• C Example: The main function of this example includes the C API, chimescalc_C.{h,cpp}, which creates a global static pointer to a chimesFF object. The chimesFF pointer object is set up, i.e. by set_chimes(), and used for access to chimesFF member functions, etc.

  • Navigate to chimesFF/examples/c

  • Compile with: make all

  • Test with: ./C_wrapper-direct_interface <parameter file> <xyz file>

  • Additional notes:

    • *.xyz files must not contain any information beyond atom type and x-, y-, and z- coordinate on coordinate lines.

    • This implementation does NOT use ghost atoms/layering thus the input system MUST have box lengths greater than two times the largest outer cutoff, or results will not be correct.

• C++ Example: The main function of this example creates an instance of serial_chimes_interface (i.e. a class inheriting chimesFF, which computes energy, per-atom forces, and stress tensor for an overall system). For additional details, see The ChIMES Calculator Serial Interface

  • Navigate to chimesFF/examples/cpp

  • Compile with: make all

  • Test with: ``./CPP-interface <parameter file> <xyz file> ``

• Fortran Example: Similar to the C example, this main function establishes a pointer to a chimesFF object via f_set_chimes(). The f_set_chimes() function call is defined in chimescalc_F.f90, a wrapper for the C API chimescalc_C.cpp (i.e which facilitates C-style access to chimesFF member functions, etc). Actual linking is achieved at compilation. See the Makefile for details.

  • Navigate to chimesFF/examples/fortran

  • Compile with: make all

  • Test with: ./fortran_wrapper-direct_interface <parameter file> <xyz file>

  • Additional notes:

    • *.xyz files must not contain any information beyond atom type and x-, y-, and z- coordinate on coordinate lines.

    • This implementation does NOT use ghost atoms/layering thus the input system MUST have box lengths greater than two times the largest outer cutoff, or results will not be correct.

• Python Example: This example accesses chimesFF functions through chimescalc_py.py`, a ctypes-based python API for access to the C API functions (i.e. through chimescalc_C.cpp). Once chimescalc_py.py is imported, it is associated with a compiled C API library file, i.e. lib-C_wrapper-direct_interface.so and can be used to access chimesFF member functions.

  • Navigate to chimesFF/examples/python

  • Compile lib-C_wrapper-direct_interface.so with: make all

  • Test with: python main.py <parameter file> <coordinate file>

  • Additional notes:

    • Requires lib-C_wrapper-direct_interface.so in the same directory, which is generated via make all

    • Expects to be run with Python version 3.X

    • This implementation does NOT use ghost atoms/layering thus the input system MUST have box lengths greater than two times the largest outer cutoff, or results will not be correct.