29 CFR § 4062.7 - Calculating interest on liability and refunds of overpayments.
(a) Interest. Whether or not the PBGC has granted deferred payment terms pursuant to § 4062.9, the amount of liability under this part includes interest, from the termination date, on any unpaid portion of the liability. Such interest accrues at the rate set forth in paragraph (c) of this section until the liability is paid in full and is compounded daily. When liability under this part is paid in more than one payment, the PBGC will apply each payment to the satisfaction of accrued interest and then to the reduction of principal.
(b) Refunds. If a contributing sponsor or member(s) of a contributing sponsor's controlled group pays the PBGC an amount that exceeds the full amount of liability under this part, the PBGC shall refund the excess amount, with interest at the rate set forth in paragraph (c) of this section. Interest on an overpayment accrues from the later of the date of the overpayment or 10 days prior to the termination date until the date of the refund and is compounded daily.
(c) Interest rate. The interest rate on liability under this part and refunds thereof is the annual rate prescribed in section 6601(a) of the Code, and will change whenever the interest rate under section 6601(a) of the Code changes.