• Code enabled by default should be standard Fortran 2008 [-std=f2008] and C99 [-std=c99]
• OpenMP code should follow version 3.X of the standard
• MPI should should follow version 3 of the standard
• Extended functionality should match POSIX and LSB.

• Every USE-statement should have an ONLY:-clause, which lists the imported symbols [-Wuse-without-only].
• Every OMP PARALLEL-region should declare default(none).
• Every static variable should be marked with the SAVE attribute.
• Every Fortran module should contain the line IMPLICIT NONE [-fimplicit-none].
• Every conversion that might change value should be explicit [-Wconversion]. Rationale:
  • INTEGER i=4.9999, i.e. i=INT(4.9999) implies i==4. Use NINT(4.9999) as appropriate.
  • natom*natom, nrow_global*ncol_global overflows INT(KIND=4) for large simulations.
  • always use REAL() with KIND parameter, i.e. r = REAL(i, KIND=dp).
  • avoid FLOAT() in favour of REAL(, KIND=dp).
  • the global number of grid points (pw_grid_type%ngpts,ngpts_cut) overflows INT(KIND=4) for large simulations.

• Do not use the GOTO-statement. See also http://xkcd.com/292/ and 10.1145/362929.362947
• Do not use left-hand-side (lhs) reallocations of allocatables [-Wrealloc-lhs]. Why?
• Do not use FORALL constructs. Why?
• Do not use OMP THREADPRIVATE variables.
• Do not query the STAT from a DEALLOCATE, the Fortran runtime takes care.
• Do not use RANDOM_NUMBER(), it's not consistent across different compilers.

There are two measure of defense against spaghetti code:

1. Decoupling on the module and package level:
   • Every module should depend on as few other modules as possible.
   • Every package should depend on as few other packages as possible.
2. Information hiding, also known as encapsulation.
   • External libraries should be wrapped within a single module or package.
   • Every module should hide its content by containing the line PRIVATE and only few public symbols.
   • Every package should hide its content by providing only a small public API through a single module.

Always prefer built-in (intrinsic) functions instead of hand-coded routines since they usually include extra numerical checks to avoid intermediate under- or overflow while still performing optimally. Examples:

• NORM2(x) instead of SQRT(x(1)**2 + x(2)**2 + x(3)**2)
• DOT_PRODUCT(x, x) instead of x(1)**2 + x(2)**2 + x(3)**2
• DOT_PRODUCT(x, y) instead of x(1)*y(1) + x(2)*y(2) + x(3)*y(3)

For many common operations there exist wrappers in CP2K to prevent usage errors and to allow for central redirections, i.e. avoid to use direct calls to external libraries in CP2K

• Use the routines from cp_files.F instead of calling OPEN and CLOSE directly.
• Use the routines from the full-matrix fm-package instead of calling BLAS or Scalapack directly.
• Use the routines from message_passing.F instead of calling MPI directly.
• Use the routines from fft_lib.F instead of calling FFT (any library) directly.
• Use the routines from machine.F to access architecture depended things like e.g. the working directory.
• Don't use UNIT=* in WRITE or PRINT statements. Instead request a unit from the logger: iw=cp_logger_get_default_unit_nr() and write only if you actually received a unit: IF(iw>0) WRITE (UNIT=iw, ,,,).
• Use CP2K's error-handling, instead of the STOP statement.

• Every line of code has to be compiled and maintained. Hence, unused variables and code poses an unnecessary burden and should be removed [-Wunused-variable -Wunused-but-set-variable -Wunused-dummy-argument].
• Sometimes it is beneficial to keep debugging code around nevertheless. Such code should be put into a IF(debug_this_module)-block, with a parameter set to .FALSE.. This way the code will stay up-to-date and easily callable.

• Each files should start with the official CP2K header.
• Each .F file should contain either a PROGRAM or a single MODULE, whose name matches the filename.
• Each module and routine should be annotated with Doxygen documentation.
• Each preprocessor flag should start with two underscores and be documented in the INSTALL-file and added to the cp2k_flags function (cp2k_info.F).
• The code should be formatted with the prettify tool by running make -j pretty.

• Every feature should be tested, with the goal of complete code coverage by regtesting .

• Every FUNCTION and SUBROUTINE should be preceded by a valid doxygen block.
• The following keywords are required: \brief, \param (for each parameter), \retval (for functions)
• The following keywords are optional: \note, \par, \date, \author
• Please run make doxify to format your doxygen comments, or generate templates where none exist
• See our doxygen pages for the result