def clusters_to_join(clusters, corr_matrix, corr_threshold):
    """
    Finds a pairs of clusters where every pair of compounds is correlated.

    :param clusters: List of existing clusters.
    :param corr_matrix: Correlation matrix storing correlation information for every pair of molecules.
    :param corr_threshold: The minimum correlation value for a pair of molecules to be considered correlated.
    :return: Pair (list) of correlated clusters or [-1,-1]
    """
    for i in range(len(clusters)):
        for j in range(i+1, len(clusters)):
            correlated = True
            for corr_i in clusters[i]:
                if not correlated: break
                for corr_j in clusters[j]:
                    if abs(corr_matrix[corr_i][corr_j]) < corr_threshold:
                        correlated = False
                        break
            if correlated: return [i, j]

    return [-1, -1]