VMS Help FORTRAN, /MATH_LIBRARY, ACCURATE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Directs the compiler to select the standard version of the math
  library routines used to implement vectorized references to the
  ATAN, COS, EXP, LOG, LOG10, SIN, SQRT and TAN intrinsics as well as
  the exponentiation operator ** for real data types.  This version
  gives the most accurate result.  For certain ranges of input
  values, the selected routine can execute more slowly than the
  alternate version specified by FAST.  This keyword is not
  negatable.