Table 2. Closing the ‘momentum triangle’ – the possible fourth-order magnetoelectric coupling terms.

Zeroth row and column correspond to two of the four coupling terms which are always time-odd. At the intersection of the rows and columns, a third time-even irrep is given with the fourth term always being P ().

	and	and
	; ; ;	;
	; ; ;	;
	; ; ;	;
	and	and
	;	; ; ;
	; ; ;	;
	and and
	;