Monkhorst Pack-grid used in calculating transmission spectrum

[OMG123]

I knew that only (ka,kb) components are used in calculating transmission spectrum
because the structure of central molecule is not periodical along kc direction.

What I haven't understood is that central molecule is usually not periodical along (ka,kb), thus can
Monkhorst Pack-grid(ka,kb) apply to calculating transmission spectrum accurately?

[Anders Blom]

It's not because of the molecule you do the k-point sampling, but because of the electrodes. So if you have 1D electrodes (nanotube, graphene nanoribbon, nanowire, etc) then you only need 1x1 k-points, but if it's a metal surface you need to properly sample the periodicity of the surface unit cell.