def tune_step_size_random_walk(step_size, acc_rate):
    """Keep acceptance rate within 25% - 40% acceptance."""
    if acc_rate < 0.001:
        step_size *= 0.1
    elif acc_rate < 0.05:
        step_size *= 0.5
    elif acc_rate < 0.25:
        step_size *= 0.9
    elif acc_rate > 0.95:
        step_size *= 10.0
    elif acc_rate > 0.75:
        step_size *= 2.0
    elif acc_rate > 0.4:
        step_size *= 1.1
    return step_size
