title: Cannot fit bayesian model with rethinking::quap for small (1e-4) parameter values
body: <p>I'm trying to calculate the posterior for <code>p</code> using <code>rethinking</code> package version 2.13:</p>
  <pre><code>rethinking::quap(
    alist(
      n ~ dbinom(N, p),
      p ~ dnorm(1e-4, 0.5e-4)    
    ),
    data = list(n = 10, N = 10e4),
    start = list(p = 10 / 10e4)
  )
  </code></pre>
  <p>It throws an error:</p>
  <blockquote>
  <p>Error in rethinking::quap(alist(n ~ dbinom(N, p), p ~ dnorm(1e-04, 5e-05)),  :
  non-finite finite-difference value [1]
  Start values for parameters may be too far from MAP.
  Try better priors or use explicit start values.
  If you sampled random start values, just trying again may work.
  Start values used in this attempt:
  p = 1e-04</p>
  </blockquote>
  <p><code>start</code> is actually the MAP.<br />
  It works for bigger <code>p</code> values.<br />
  How to solve the problem?</p>