Continuing the discussion from Custom fix does not compute scalar?:
sorry this might be way too late but im documentating my solution for future googlers:
you need to set scalar_flag and extscalar in your Fix subclass constructor:
scalar_flag = 1;
extscalar = 0;
from fix.h:
int scalar_flag; // 0/1 if compute_scalar() function exists
int vector_flag; // 0/1 if compute_vector() function exists
int array_flag; // 0/1 if compute_array() function exists
int size_vector; // length of global vector
int size_array_rows; // rows in global array
int size_array_cols; // columns in global array
int size_vector_variable; // 1 if vec length is unknown in advance
int size_array_rows_variable; // 1 if array rows is unknown in advance
int global_freq; // frequency s/v data is available at
int extscalar; // 0/1 if global scalar is intensive/extensive
int extvector; // 0/1/-1 if global vector is all int/ext/extlist
int *extlist; // list of 0/1 int/ext for each vec component
int extarray; // 0/1 if global array is intensive/extensive
