  A modified density matrix renormalization group (DMRG) algorithm is applied
to the zigzag spin-1/2 chain with frustrated antiferromagnetic exchange $J_1$,
$J_2$ between first and second neighbors. The modified algorithm yields
accurate results up to $J_2/J_1 \approx 4$ for the magnetic gap $\Delta$ to the
lowest triplet state, the amplitude $B$ of the bond order wave (BOW) phase, the
wavelength $\lambda$ of the spiral phase, and the spin correlation length
$\xi$. The $J_2/J_1$ dependences of $\Delta$, $B$, $\lambda$ and $\xi$ provide
multiple comparisons to field theories of the zigzag chain. The twist angle of
the spiral phase and the spin structure factor yield additional comparisons
between DMRG and field theory. Attention is given to the numerical accuracy
required to obtain exponentially small gaps or exponentially long correlations
near a quantum phase transition.
