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Fortran API

Fortran API#

The tblite library seamlessly integrates with other Fortran projects via module interfaces,

Note

Generally, all quantities used in the library are stored in atomic units.

Handling of geometries and structure#

The basic infrastructure to handle molecular and periodic structures is provided by the modular computation tool chain library. The library provides a structure type which is used to represent all geometry related informations in tblite. A structure type can be constructed from arrays or read from a file.

The constructor is provided with the generic interface new and takes an array of atomic numbers (integer) or element symbols (character(len=*)) as well as the cartesian coordinates in Bohr. Additionally, the molecular charge and the number of unpaired electrons can be provided the charge and uhf keyword, respectively. To create a periodic structure the lattice parameters can be passed as 3 by 3 matrix with the lattice keyword.

An example for using the constructor is given here

subroutine example
   use mctc_env, only : wp
   use mctc_io, only : structure_type, new
   implicit none
   type(structure_type) :: mol
   real(wp), allocatable :: xyz(:, :)
   integer, allocatable :: num(:)

   num = [6, 1, 1, 1, 1]
   xyz = reshape([ &
     &  0.00000000000000_wp, -0.00000000000000_wp,  0.00000000000000_wp, &
     & -1.19220800552211_wp,  1.19220800552211_wp,  1.19220800552211_wp, &
     &  1.19220800552211_wp, -1.19220800552211_wp,  1.19220800552211_wp, &
     & -1.19220800552211_wp, -1.19220800552211_wp, -1.19220800552211_wp, &
     &  1.19220800552211_wp,  1.19220800552211_wp, -1.19220800552211_wp],&
     & [3, size(num)])

   call new(mol, num, xyz, charge=0.0_wp, uhf=0)

   ! ...
end subroutine example

To interact with common input file formats for structures the read_structure procedure is available. The file type is inferred from the name of the file automatically or if a file type hint is provided directly from the enumerator of available file types. The read_structure routine can also use an already opened unit, but in this case the file type hint is mandatory to select the correct format to read from.

subroutine example
   use mctc_env, only : error_type
   use mctc_io, only : structure_type, read_structure, file_type
   implicit none
   type(structure_type) :: mol
   type(error_type), allocatable :: error
   character(len=:), allocatable :: input

   input = "struc.xyz"

   call read_structure(mol, input, error, file_type%xyz)
   if (allocated(error)) then
      print '(a)', error%message
      stop 1
   end if

   ! ...
end subroutine example

The structure type as well as the error type are using only allocatable members and can therefore be used without requiring explicit deconstruction.

Certain members of the structure type should be considered immutable, like the number of atoms (nat), the identifiers for unique atoms (id) and the boundary conditions (periodic). To change those specific structure parameters the structure type and all dependent objects should be reconstructed to ensure a consistent setup. Other properties, like the geometry (xyz), molecular charge (charge), number of unpaired electrons (uhf) and lattice parameters (lattice) can be changed without requiring to reconstruct dependent objects like calculators or restart data.

Error handling#

The basic error handler is an allocatable derived type, available from mctc_env as error_type, which signals an error by its allocation status.

use mctc_env, only : error_type, fatal_error
implicit none
type(error_type), allocatable :: error

call always_ok(error)
if (allocated(error)) then
   print '(a)', "Unexpected failure:", error%message
end if

call always_failed(error)
if (allocated(error)) then
   print '(a)', "Error:", error%message
end if

contains
   subroutine always_ok(error)
      type(error_type), allocatable, intent(out) :: error
   end subroutine always_ok

   subroutine always_failed(error)
      type(error_type), allocatable, intent(out) :: error

      call fatal_error(error, "Message associated with this error")
   end subroutine always_failed
end

An unhandled error might get dropped by the next procedure call.

Calculation context#

The calculation context is available with the context_type from the tblite_context module. The context stores error messages generated while running which can be queried using the type bound function failed. To access the actual errors the messages can be removed using the type bound subroutine get_error.

An output verbosity is available in the context as the member verbosity, all procedures with access to the context will default to the verbosity of the context unless the verbosity level is overwritten by an argument. To cutomize the output the context_logger abstract base class is available. It must implement a type bound message procedure, which is used by the context to create output. This type can be used to create callbacks for customizing or redirecting the output of the library.

High-level interface#

The high-level interface is defined by the calculation context, the calculator instance and its restart data. The calculation context is defined with the context_type, which stores general settings regarding the overall method independent setup of the calculation. The actual parametrisation data is stored in the xtb_calculator type. An instance of the calculator can be used in a thread-safe way to perform calculations for a specific structure (defined by its number of atoms, unique elements and boundary conditions). Changing the specific structure parameters requires to reconstruct the calculator. Finally the specific persient data for a geometry is stored in a wavefunction_type, which allows to restart calculations based on previous results.

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