Is it permissible to place silver coins under the doorstep of a house for optimism or to bring good fortune?

If your father's action was for the purpose of optimism, then it is not the legitimate optimism, for optimism is "the good word one of you hears." However, if his intention was to preserve the money, then there is no harm, but he must pay zakat (definition) on this money annually if it reaches the nisab (definition) by itself or by combining it with other funds. Otherwise, it would be a treasure (كنز) for which its owner deserves the warning mentioned in His (Allah's) saying: "And those who hoard gold and silver and spend it not in the way of Allah – give them tidings of a painful punishment."