If the amount of tax computed by the Department is greater than the amount theretofore assessed, the excess shall be assessed by the Tax Commissioner and notice of the same shall be mailed to the taxpayer. The taxpayer shall remit such additional tax to the Department within thirty days from the date of such notice. In such case, if the return was made in good faith and the understatement of the amount in the return was not due to any fault of the taxpayer, there shall be no penalty on the additional tax because of such understatement, but interest shall be added to the amount of the deficiency at a rate determined in accordance with § 58.1-15, from the time the return was required by law to be filed until paid. If the understatement is false or fraudulent with intent to evade the tax a penalty of 100 percent shall be added together with interest on the tax at a rate determined in accordance with § 58.1-15, from the time the return was required by law to be filed until paid. Nothing contained in this section shall prevent the taxpayer from applying for a correction of any assessment as provided in Chapter 18, Article 2 (§ 58.1-1820 et seq.) of this title. The taxes imposed by this chapter shall be assessed within three years from the date on which such taxes became due and payable, except that in the case of a false or fraudulent return with intent to evade payment of such taxes, or a failure to file a return, the taxes may be assessed at any time within six years from such date.