This code will work. I've only included the first two columns you want to add.
dt = current data table();
for (i = 1, i <= nrows(dt), i++,
Column(dt, "Total Sum" )[i] = Column(dt, "Dimer [Cl5Si2-O-Si2Cl5]" )[i]
+ Column(dt, "Silicon Tetrachloride(SiCl4)" )[i];
);