  We present a fast method of producing mock galaxy catalogues that can be used
to compute covariance matrices of large-scale clustering measurements and test
the methods of analysis. Our method populates a 2nd-order Lagrangian
Perturbation Theory (2LPT) matter field, where we calibrate masses of dark
matter halos by detailed comparisons with N-body simulations. We demonstrate
the clustering of halos is recovered at ~10 per cent accuracy. We populate
halos with mock galaxies using a Halo Occupation Distribution (HOD)
prescription, which has been calibrated to reproduce the clustering
measurements on scales between 30 and 80 Mpc/h. We compare the sample
covariance matrix from our mocks with analytic estimates, and discuss
differences. We have used this method to make catalogues corresponding to Data
Release 9 of the Baryon Oscillation Spectroscopic Survey (BOSS),producing 600
mock catalogues of the "CMASS" galaxy sample. These mocks enabled detailed
tests of methods and errors that formed an integral part of companion analyses
of these galaxy data.
