private double[] randomSampler() {
        int paras = parameterNames.length;
        double[] sample = new double[paras];
        for (int i = 0; i < paras; i++) {
            double d = custom_rand();
            sample[i] = (lowBound[i] + d * (upBound[i] - lowBound[i]));
        }
        return sample;
    }