Coding Style

The programming style of DL_POLY is intended to ensure the code is a consistent, readable and maintainable as possible. The following rules apply throughout the code. Contributors to the code are urged to read and apply this style before submitting code for inclusion consideration.


All routines employ DL_POLY internal units. Output contains, when relevant, the units.


simulation temperature (K)          1.0000E+01

simulation pressure (katms)         0.0000E+00

Integration : Leapfrog Verlet
Ensemble : NVT Nose-Hoover
thermostat relaxation time (ps)     1.0000E-01

selected number of timesteps         5000

General Style

  • Use modern Fortran free form syntax.

  • Code should be written in Fortran2003/2008 dialect if possible.

  • Check the deprecated features from Fortran 2003/2008 and prior. Avoid using these features.

  • Fortran2008 should be treated with care, since not all compilers implement it.

  • Do not use Common blocks and Block Data, use Modules with public data if constant or a user defined type to group data which changes during runtime.

  • Do not use go to statements

  • Do not use format statements

  • File extension shall be .F90 for any new written code.

  • Indent code blocks by two space characters. Do not use tabs as they are not part of the Fortran standard.

Subroutine init(T,k,traj)
  Class(current_type) :: T
  Type(trajectory_type),Intent(in)  :: traj
  Real(Kind=dp),Intent(in)          :: k(3)

  T%k = k
  T%n = traj%nFrames
End Subroutine init
  • Indent Contains statements to the same level as the sorrounding Module, or procedure

Module my_module
  Implicit None

  Integer( Kind = wi ), Parameter :: pi = 3.14...



  Subroutine my_subroutine(a,b)


End Module my_module
  • Do not use more than one blank line to separate blocks of code.

  • Code lines shall not exceed 132 characters (the modern Fortran standard limit).

  • Do not write Fortran keywords in ALL capitals. Capitalize the first letter of the statement to make them stand out for the reader.

Program dl_poly
  Use kinds, Only : wi,wp
  Implicit None
End Program dl_poly
  • While Fortran supports multiple statements on a line separated by ; try to use them sparingly and wisely.

  • Variables, constants and program units should be named in English or using common physics notation. Do not use cryptic names.

  • Try to group variables names based on physical meaning or usage.

! Verlet neighbour list data
!> Update cells flag
Logical, Public :: update = .true.
!> Unconditional update flag
Logical, Public :: unconditional_update = .false.

!> Tracking points for Verlet neighbour list
Real( Kind = wp ), Allocatable, Public :: xbg(:),ybg(:),zbg(:)

!> Largest vdw cutoff, defines Verlet neighbour list radius
Real( Kind = wp ), Public :: cutoff
!> Padding around cutoff
Real( Kind = wp ), Public :: padding
!> Actual Verlet neighbour list cutoff (cutoff+padding)
Real( Kind = wp ), Public :: cutoff_extended

!> Linked cell list
Integer( Kind = wi ), Allocatable, Public :: list(:,:)
!> Maximum rank of linked cell list
Integer( Kind = wi ), Public :: max_list
!> Maximum number of cells per domain
Integer( Kind = wi ), Public :: max_cell
  • Avoid naming program units, variables, constants using intrinsincs routines or keywords from fortran standard.

  • While Fortran is case insensitive, use for lower case letters for program units, variables and constants.

  • Where more than a word is used in a name use _ as separator (snake case notation).

  • Use the mordern syntax for logical expressions,

    • == not .eq.

    • /= not .ne.

    • > not .gt.

    • < not .lt.

    • >= not .ge.

    • <= not .le. .

  • Prefer positive testing in logical blocks.

  If (isw == 0) Then
     ! do some fancy code
    ! do some not so fancy code
  End If
! to
  If (isw /= 0) Then
    ! do some not so fancy code
    ! do some fancy code
  End If
  • One line If shall be avoided.

  • Always use the optional separation space for End constructs, e.g. End If not Endif and End Do not Enddo

  • For program units use the full End: End Subroutine name.

  • Never use Go To statements

  • Use DoEnd Do instead of Do … Label Continue

  • Do not use Format.

  Write(*,20) reclen

20 Format('(',i0,'a)')

! replace with

  • Do not use interactive commands, like Read from keyboard or Pause.

  • Use Implicit None to avoid unwanted side effects. This is only nessecary once per module.

  • Avoid implicit casting of data: use 2.0_wp instead of 2.0 in an expression.

 If (sw == 1) Then
   factor = 2.0_wp*engke*rf
 End If
! and not
If (sw == 1) factor = 2.0_wp*engke*rf
  • Floating point comparisons shall be done with care. Consider the following examples using a tolerance of Epsilon(a) or Tiny(a),

    • a > tolerance instead of a > 0.0

    • a - b > tolerance instead of a > b

    • abs(a - b) < tolerance instead of a == b .

  • Avoid use of magic numbers, declare constants at the top level of a module and use those instead.

If (integrator == 1 ) Then
! instead use

Integer( Kind = wi ), Parameter :: VELOCITY_VERLET = 1


If (integrator == VELOCITY_VERLET ) Then
  • Any new feature shall be turnable on/off from CONTROL file.

  • Any new feature shall be documented in the manual and will cite relevant literature.

Modular structure

  • All subroutines/functions shall be enclosed by a module.

  • Modules may contain the following:

  • Type declarations

  • Subroutines and functions

  • Paramter definitions (using the Parameter attribute)

  • Interfaces for overloaded procedures

  • Declaration of Public data

  • Modules may NOT contain the following:

  • Variables (i.e. specifications without the Parameter attribute)

  • By default everything in a module shall be made private, explicitly make public what is needed outside the module or type using the Public statement.

  • Data which is used only in the defining module should be declared private.

  • Each module should be in a separate file.

  • Module names shall match their file names.

  • When using a module with the Use statement, Only must also be used

  • While overloading operators may be tempting, it is best avoided if you prefer performance to aesthetical beauty.

Use domains_module, Only : map
  • User derived types should be created to contain data relevant to the module, preferably one per module.

  • Types may match the module name but append them with **_type**.

  • Types shall provide init and Final procedures, optionally a summary procedure.

  • If provided a summary procedure shall produce valid YAML 1.2 output.

Specification Statements

  • Align declaration statements clearly separating types and attributes from names.

  • Separate variable declaration block from code block of a subroutine by a blank line.

  • Use separate declaration blocks for routine arguments and local variables.

!> Calculate the factorial of n
Function factorial(n) Result(res)
  !> The integer to calculate the factorial of
  Integer( Kind = wi ), Intent( In    ) :: n
  !> The factorial of n
  Integer( Kind = wi ) :: res

  !> Running total
  Integer( Kind = wi ) :: total
  !> Loop iterator
  Integer( Kind = wi ) :: i

  • Always use :: to separate types and attributes from names.

  • Use :: as an alignment guide if the list of names is too long.

  • Separate logical declaration blocks can be aligned differently to save screen real estate, e.g. parameters vs internal variables of a routine.

Integer,           Intent( In    ) :: imcon,mxshak
Real( Kind = wp ), Intent( InOut ) :: xxx(1:mxatms),yyy(1:mxatms),zzz(1:mxatms)
Real( Kind = wp ), Intent(   Out ) :: strcon(1:9)
Real( Kind = wp ), Intent(   Out ) :: vircon

Logical           :: safe
Integer           :: fail(1:2),i,j,k,icyc
Real( Kind = wp ) :: amti,amtj,dli,dlj, &


  • Comments shall be indented to align with the code

  • Comments shall be written in lower case except for proper nouns and standard abreviations.

  • Comments shall explain what a code does and why, not how it does it. Let the code explain how it is done.

Ford and Doxygen

  • By conforming to the following style useful developer documentation may be created automatically using either FORD or Doxygen.

  • Comments attached to program units, variables and derived types may be automatically documented.

  • Documentation must precede the declaration of the unit, variable or derived type.

  • Comments to be documented must use the tag “!>” (This is default tag in FORD for a comment preceding the content. In Doxygen, “!>” is the only tag which can both start and continue a comment. So this seems to be the best compromise to make the source compatible with both systems. FORD does not like inline comments).

  • To insert a line break in a multiline comment use a blank comment line.

  • Comments use markdown syntax for formatting.

  • LaTeX style maths may be used to include equations in the documentation.

  • See the example structure for more comprehensive examples of documentation comments.

!># Example
!> Author - John Smith
!> An example program
Program example
  Use kinds, Only : wi
  Implicit None

  !> Integer counter
  Integer( Kind = wi ) :: i


  !> Calculate the factorial of n
  Function fact(n)



  • A Function shall be pure (with no side-effects). If side-effect are needed use a Subroutine.

  • All arguments of a Function/Subroutine shall have an Intent attribute (with the exception of the Class in type bound procedures).

  • Avoid using Recursive procedures.

  • When you are passing an array argument to a Subroutine/Function, and the Subroutine/Function does not change the size of the array, you should pass it as an assumed shape array. Memory management of such an array is automatically handled by the Subroutine/Function, and you do not have to worry about having to Allocate or Deallocate your array. It also helps the compiler to optimise the code.

Allocatable Data and Pointers

  • If possible Allocate and Deallocate for an array or pointer shall appear in the same scope.

  • For Allocatable members of a user defined type, allocation shall happen in the init and deallocation in the final subroutine.

  • In all cases, Deallocate in the opposite order you did Allocate.

Allocate(xxx(n),Stat = stat)
Allocate(yyy(n),Stat = stat)
Allocate(zzz(n),Stat = stat)

  • If using Pointer, define it before usage by pointing to Null or using Nullify.

  • Similarly, when a pointer is not used anymore nullify it using the same techniques as above.