kim-api  2.3.1-git+v2.3.0-git-2-g378406f9.GNU.GNU.
An Application Programming Interface (API) for the Knowledgebase of Interatomic Models (KIM).

Previous Section: Theory for the Portable Model Interface.

The KIM API Portable Model Interface (KIM API/PMI)

In code, Portable Models (PMs) and Model Drivers (MDs) consist of up to eight routines which perform specific tasks.

  1. The ModelCreate (or ModelDriverCreate ) routine (required), which performs initialization tasks for the KIM::Model object.
  2. The ModelComputeArgumentsCreate routine (required), which performs initialization tasks for a KIM::ComputeArguments object.
  3. The ModelCompute routine (required), which uses the configuration information stored in a KIM::ComputeArguments object to perform the PM's core computational tasks.
  4. The ModelExtension routine (optional), which provides a mechanism for creating and using non-standard extensions to the KIM API.
  5. The ModelRefresh routine (required if parameter pointers are set, otherwise should not be provided), which performs updates after a simulator makes changes to the PM's parameters (if this is supported).
  6. The ModelWriteParameterizedModel (optional) routine, which can be used to write the parameter files and CMake file necessary to create a new parameterized PM from the current set of in-memory parameters.
  7. The ModelComputeArgumentsDestroy routine (required), which performs finalization tasks for a KIM::ComputeArguments object.
  8. The ModelDestroy routine (required), which performs finalization tasks for the KIM::Model object.

The KIM API/PMI provides a separate interface to be used by the PM or MD. For each of the routines itemized above, the following table lists (i) the associated KIM::ModelRoutineName extensible enumeration constant, (ii) links to the associated KIM API/PMI interface(s) available within the routine, (iii) links to the routine's prototype (in C++, C, and Fortran), and (iv) whether the KIM API/PMI requires a PM or MD to provide the routine, or if it is optional.

Model Routine Name constant KIM API/PMI Interface Routine prototype (C++; C; Fortran) KIM API/PMI Required / Optional
KIM::MODEL_ROUTINE_NAME::Create KIM::ModelCreate or
KIM::MODEL_ROUTINE_NAME::ComputeArgumentsCreate KIM::ModelComputeArgumentsCreate KIM::ModelComputeArgumentsCreateFunction;
KIM::MODEL_ROUTINE_NAME::Compute KIM::ModelCompute and
KIM::MODEL_ROUTINE_NAME::Extension KIM::ModelExtension KIM::ModelExtensionFunction;
KIM::MODEL_ROUTINE_NAME::Refresh KIM::ModelRefresh KIM::ModelRefreshFunction;
required if one or more parameter pointers set, otherwise should not be provided
KIM::MODEL_ROUTINE_NAME::WriteParameterizedModel KIM::ModelWriteParameterizedModel KIM::ModelWriteParameterizedModelFunction;
KIM::MODEL_ROUTINE_NAME::ComputeArgumentsDestroy KIM::ModelComputeArgumentsDestroy KIM::ModelComputeArgumentsDestroyFunction;
KIM::MODEL_ROUTINE_NAME::Destroy KIM::ModelDestroy KIM::ModelDestroyFunction;

The above table also indicates which routines must be provided by a PM. For optional routines, each PM must indicate that the routine is required or optional for use by the simulator, as described below.

Language restrictions implied by the KIM API/PMI:
The KIM API/PMI implements a cross-language object-oriented-like framework. All persistent data (analogous to member variables) needed by a PM or MD object must be stored using the Caching Capabilities of the KIM API. For the framework to operate correctly, without surprising behavior, PM and MD implementations must obey certain restrictions as described below.
  • C++

    The use of static and/or global variables within PM and MD code must be avoided.

  • C

    The use of static and/or global variables within PM and MD code must be avoided.

  • Fortran

    The use of COMMON blocks must be avoided. The use of the SAVE attribute must be avoided. Thus, variables should not be initialized when they are declared (as this implies the SAVE attribute). Further, the use of module variables, except for those defined with the PARAMETER attribute, must be avoided. (The Fortran 2008 standard requires that all module variables, implicitly, have the SAVE attribute; that is, they are stored in static memory.) Similarly, all Fortran subroutines and functions should have the RECURSIVE attribute (this is the default starting only with the Fortran 2015 standard). This is because without the RECURSIVE attribute compilers are allowed to use static memory for a subroutine's/function's local variables (effectively giving them the SAVE attribute).

    These restrictions are even more important if thread-safety is required. The use of RECURSIVE subroutines/functions and avoidance of using the SAVE attribute help to assure that memory access conflicts are averted.

The KIM API/PMI provides two interfaces to be used by the Simulator, KIM::Model and KIM::ComputeArguments, for interacting with a PM. The interaction between the simulator and a PM involves the following steps:

The KIM API/PMI provides a list of all compute-arguments and compute-callbacks defined as part of the official API. Each argument and callback has a "Support Status" that can be one of four values: requiredByAPI, notSupported, required, or optional. A PM specifies a support status value, as part of its ModelComputeArgumentsCreate routine, for every compute-argument and compute-callback defined by the KIM API/PMI. Some arguments and callbacks are required by the KIM API and can only have a support status of requiredByAPI. All other arguments and callbacks are not required by the KIM API/PMI, and therefore the PM may set their support status to any one of the three remaining options: required, optional, or notSupported. Just before the PM's ModelComputeArgumentsCreate routine is executed, the KIM API/PMI initializes the support status of all arguments and callbacks to either requiredByAPI or notSupported, as indicated in the below tables. It is the responsibility of the simulator to use the KIM::ComputeArguments object interface to determine the support status of each compute-argument and compute-callback and to use this information to determine if the PM is capable of performing the desired computation.

Below, lists of each input compute-argument, output compute-argument, and compute-callback are provided. To be explicit, zero-based particle numbering is used where necessary.

Input compute-argument table:

Compute Argument Name Unit Data Type Extent Memory Layout Valid Support Statuses (bold – default)
numberOfParticles N/A integer 1 requiredByAPI
particleSpeciesCodes N/A integer numberOfParticles \(sc^{(0)}, sc^{(1)}, \dots\) requiredByAPI
particleContributing N/A integer numberOfParticles \(c^{(0)}, c^{(1)}, \dots\) requiredByAPI
coordinates length double numberOfParticles * 3 \(r^{(0)}_1, r^{(0)}_2, r^{(0)}_3, r^{(1)}_1, r^{(1)}_2, \dots\) requiredByAPI

Output compute-argument table:

Compute Argument Name Unit Data Type Extent Memory Layout Valid Support Statuses (bold – default)
partialEnergy energy double 1 required, optional, notSupported
partialForces force double numberOfParticles * 3 \(f^{\mathcal{C}(0)}_1, f^{\mathcal{C}(0)}_2, f^{\mathcal{C}(0)}_3, f^{\mathcal{C}(1)}_1, f^{\mathcal{C}(1)}_2\dots\) required, optional, notSupported
partialParticleEnergy energy double numberOfParticles \(E^{\mathcal{C}}_0, E^{\mathcal{C}}_1, E^{\mathcal{C}}_2, \dots\) required, optional, notSupported
partialVirial energy double 6 \(V^{\mathcal{C}}_{11}, V^{\mathcal{C}}_{22}, V^{\mathcal{C}}_{33}, V^{\mathcal{C}}_{23}, V^{\mathcal{C}}_{31}, V^{\mathcal{C}}_{12}\) required, optional, notSupported
partialParticleVirial energy double numberOfParticles * 6 \(\mathbf{V}^{\mathcal{C}(0)}, \mathbf{V}^{\mathcal{C}(1)}, \mathbf{V}^{\mathcal{C}(2)}, \dots\) required, optional, notSupported

Compute-callback table:

Compute Callback Name Valid Support Statuses (bold – default)
GetNeighborList requiredByAPI
ProcessDEDrTerm required, optional, notSupported
ProcessD2EDr2Term required, optional, notSupported

See the documentation of the KIM::Model and KIM::ComputeArguments interfaces for more details of the KIM API/PMI from the simulator's perspective.

See the documentation of the KIM::ModelCompute, KIM::ModelComputeArguments, KIM::ModelComputeArgumentsCreate, KIM::ModelComputeArgumentsDestroy, KIM::ModelCreate, KIM::ModelDestroy, KIM::ModelDriverCreate, KIM::ModelExtension, KIM::ModelRefresh, and KIM::ModelWriteParameterizedModel interfaces for more details of the KIM API/PMI from the PM's perspective.

The KIM API Simulator Model Interface (KIM API/SMI)

In code, Simulator Models (SMs) are simply a set of parameter files and a specification file which contains simulator input commands and metadata (supported species, units, etc.) needed to run the model in its native simulator.

The KIM API/SMI consists of the KIM::SimulatorModel interface which provides programatic access to the SM's parameter files and the contents of its specification file. Conceptually, an SM specification file contains a series of key-value pairs. Some key-value pairs are defined and required by the KIM API/SMI. All other key-value pairs are called "Simulator Fields". For flexibility and generality, the content and meaning of the simulator fields is not defined by the KIM API/SMI. Instead, each simulator is free to create and define its own set of specifications. The value part of a simulator field key-value pair is one or more strings called "Simulator Field Lines". These strings may contain template tags (of the form "@<template-tag-key>@") which the KIM::SimulatorModel object will replace by performing template substitution using the "Template Map". The KIM API/SMI defines a small set of template mappings which facilitate providing copies of the SM's parameter files to the simulator. Additionally, simulators may define their own template mappings and add these to the KIM API/SMI's set to be used for template substitution.

Although the KIM API/SMI is designed for maximum flexibility, an example of one way it can be used by a simulator to support SMs is helpful. In particular, for the LAMMPS simulator, performing a simulation with a specific model typically involves three separate parts: (1) specifying the physical units in which the model's parameters are given and therefore the units in which the simulation will be performed; (2) specifying other default properties needed by the model which must be set before the LAMMPS simulation box is defined, such as the LAMMPS atom style; and (3) specifying the pair_style or other style commands that define the model's interactions and read in the model's parameter files. Here is an example LAMMPS input file for the ReaxFF potential, similar to the CHO example included in the LAMMPS distribution.

# REAX potential for CHO system
units real
atom_style charge
neigh_modify every 10 delay 0 check no
neighbor 2 bin
read_data data.CHO
pair_style reax/c lmp_control
pair_coeff * * ffield.reax.cho H C O
fix reaxqeq all qeq/reax 1 0.0 10.0 1e-6 param.qeq
fix 1 all nve
fix 2 all temp/berendsen 500.0 500.0 100.0
timestep 0.25
run 100

In this case, the units line corresponds to (1) above. The atom_style and neigh_modify lines correspond to (2) above. And, the pair_style, pair_coeff, and fix reaxqeq lines correspond to (3) above. The strings "lmp_control", "ffield.reax.cho", and "param.qeq" are parameter file names (which are expected to be in the current working directory, since the names do not start with a '/'). Also of note is the string of parameters "H C O" on the pair_coeff line, which specifies a mapping between the species names "H", "C", and "O" used by the ReaxFF model and the LAMMPS atom type numbers 1, 2, and 3, respectively. The number of atom types and their species mapping are part of the simulation, not part of the model. In this case these values are specified within the "data.CHO" input file listed on the read_data line. Since the number of atom types and the species map can change from simulation to simulation, any LAMMPS KIM SM implementation will need a way of specifying the atom type mapping on the pair_coeff line.

The LAMMPS SM implementation accommodates all of these needs by defining three Simulator Fields, corresponding to the three parts of a LAMMPS model specification discussed above. (1) A "units" field, with a single line containing the LAMMPS unit system string required by the model. (2) A "model-init" field, with zero or more lines containing, in this case, the atom_style and neigh_modify commands. And (3) a "model-defn" field, with zero or more lines containing, in this case, the pair_style, pair_coeff, and fix reaxqeq commands. In the latter field lines, the correct parameter file names are obtained through the KIM API/SMI's template substitution mechanism, using the standard template keys for the SM's parameter files. The correct atom type mapping is obtained through a special, LAMMPS defined, template map with key "atom-type-sym-list". The result of all of these considerations and definitions is the following SM specification file (see the KIM API/SMI Specification File Schema for details of the file format) for the LAMMPS ReaxFF model.

"kim-api-sm-schema-version" 1
"model-name" "Sim_LAMMPS_ReaxFF_ChenowethVanDuinGoddard_2008_CHO__SM_584143153761_000"
"simulator-name" "LAMMPS"
"simulator-version" "28 Feb 2019"
"supported-species" "C H O"
"units" "real"
"model-init" [ "atom_style charge"
"neigh_modify one 4000"
"model-defn" [ "pair_style reax/c @<parameter-file-2>@ safezone 2.0 mincap 100"
"pair_coeff * * @<parameter-file-1>@ @<atom-type-sym-list>@"
"fix reaxqeq all qeq/reax 1 0.0 10.0 1.0e-6 @<parameter-file-3>@"

The ordering of the SM's parameter files is specified in the SM's CMakeLists.txt configuration file. With this SM defined and installed in one of the KIM API collections, the following is equivalent to the above LAMMPS input script.

# REAX potential for CHO system
kim_init Sim_LAMMPS_ReaxFF_ChenowethVanDuinGoddard_2008_CHO__SM_584143153761_000 real
neighbor 2 bin
read_data data.CHO
kim_interactions C H O
fix 1 all nve
fix 2 all temp/berendsen 500.0 500.0 100.0
timestep 0.25
run 100

See the documentation of the KIM::SimulatorModel interface for more details of the KIM API/SMI.

Next Section: Summary of Differences Between kim-api-v1 and kim-api-v2.