On Wed, 17 Jun 2015, Procházka LukᨠIng. - Pontex s. r. o. wrote:

> Hello,
>
> thanks for the solution: \define\dif{\mathop{}\!\mfunction{d}}

Another alternative is (untested)

\definemathcommand[dif][nolop]{\mfunction{d}\kern\zeropoint}

> Some explanation for \mathop and \mfunction would be appreciated,
> and also explanation why \definemathcommand[dif][nolop]{\mfunction{d}} didn't 
> work...

Roughtly, the above exapnds to

\define\dif{\mathop{\mfunction{d}}\nolimits}

When the argument of \mathop is a single character (d, in this case), 
mathop centers the charater on the 'math-axis'. The rationale is that such 
a behavior is desirable when defining single letter math operators such as 
sum, int, bigcap, bigcup, etc. Adding \kern\zeropoint makes TeX think that 
the argument of \mathop is more than a single character, so it does not 
center d along the math-axis.

Aditya