  Using quantum field theory and bosonization, we determine the quantum phase
diagram of the one-dimensional Hubbard model with bond-charge interaction $X$
in addition to the usual Coulomb repulsion $U$ at half-filling, for small
values of the interactions. We show that it is essential to take into account
formally irrelevant terms of order $X$. They generate relevant terms
proportional to $X^2$ in the flow of the renormalization group (RG). These
terms are calculated using operator product expansions. The model shows three
phases separated by a charge transition at $U=U_c$ and a spin transition at
$U=U_s>U_c$. For $U<U_c$ singlet superconducting correlations dominate, while
for $U>U_s$, the system is in the spin-density wave phase as in the usual
Hubbard model. For intermediate values $U_c<U<U_s$, the system is in a
spontaneously dimerized bond-ordered wave phase, which is absent in the
ordinary Hubbard model with $X=0$. We obtain that the charge transition remains
at $U_c=0$ for $X \neq 0$. Solving the RG equations for the spin sector, we
provide an analytical expression for $U_s(X)$. The results, with only one
adjustable parameter, are in excellent agreement with numerical ones for $X <
t/2$ where $t$ is the hopping.
