Perfluorobutane is an inert, high-density colorless gas and has a high Global Warming Potential value of 4800.
At about the same temperature of -1.7 ⁰C, it has a density of 11.2 kg/m3, opposed to 2 kg/m3 for CO2 (both at 1 atm.)
Edit: Perflenapent (C5F12) has a molar mass of 288 g/mol, opposed to perfluorobutane with 238 g/mol, and according to this saturation curve could still be a gas at 25 kPa and 0⁰ C.
So this gas could cause a (considerable) higher pressure.
So if we ever will have serious intentions to terraform Mars, here are the gases to start with!
But first we could begin in a modest way, for instance with Ganges Cavus, a collapse feature lying in the eastern part of the Valles Marineris system of canyons.
From the edges to its floor it is about 5 km deep (calculated with Mars Trek), and calculated with the Mars Atmosphere Model the atmospheric pressure at the edge would be 0.692 kPa.
With that pressure (of the CO2 atmosphere) as a starting point at its edges, what would be the pressure at the floor of Ganges Cavus, if it was filled with perfluorobutane?
Liquid water of 15⁰ C would need a pressure of 1.8 kPa, water of 20⁰ C would need 2.4 kPa. (Steamtablesonline)
10⁰ C would need 1.3 kPa.