How to do the layout simulation of the mixed-signal circuit

I am developing a mixed signal circuit. The layout of the analog part which contais the passive components will be done manually. However, I am using the digital library which does not contain the netlist or layout informaiton. The layout will be instantiate when fabricating the chip. How can I do the simulation of layour of the whole circuit before submit it to fabricate?

Thanks.

You can use momentum to help the layout simulations.

...so what is momentum?