  Laser Interferometer Space Antenna (LISA) will routinely observe coalescences
of supermassive black hole (BH) binaries up to very high redshifts. LISA can
measure mass parameters of such coalescences to a relative accuracy of
$10^{-4}-10^{-6}$, for sources at a distance of 3 Gpc. The problem of parameter
estimation of massive nonspinning binary black holes using post-Newtonian (PN)
phasing formula is studied in the context of LISA. Specifically, the
performance of the 3.5PN templates is contrasted against its 2PN counterpart
using a waveform which is averaged over the LISA pattern functions. The
improvement due to the higher order corrections to the phasing formula is
examined by calculating the errors in the estimation of mass parameters at each
order. The estimation of the mass parameters ${\cal M}$ and $\eta$ are
significantly enhanced by using the 3.5PN waveform instead of the 2PN one. For
an equal mass binary of $2\times10^6M_\odot$ at a luminosity distance of 3 Gpc,
the improvement in chirp mass is $\sim 11%$ and that of $\eta$ is $\sim 39%$.
Estimation of coalescence time $t_c$ worsens by 43%. The improvement is larger
for the unequal mass binary mergers. These results are compared to the ones
obtained using a non-pattern averaged waveform. The errors depend very much on
the location and orientation of the source and general conclusions cannot be
drawn without performing Monte Carlo simulations. Finally the effect of the
choice of the lower frequency cut-off for LISA on the parameter estimation is
studied.
