I am running an Abaqus riks analysis to calculate critical buckling load for a Hollow steel column. I ran the analysis when Nlgeom is ON and OFF for the analysis Step and i am getting different results for the ciritical buckling. Therefore, I would like to know the effect of the Nlgoem, and when to turned On and OFF and what is the criterial or theory that allows you to turn it ON or OFF when analyzing critical buckling of columns.